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Security Hacks

6 Clever Ways to Find Out If Someone Has Opened Your Door (With Illustration)

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An open door

The idea that a person secretly enters your private space while you’re away, can be pretty frightening and disturbing for many of us.

If you want to find out if someone has opened a door (be it a flatmate, a family member, or a burglar), you have to outsmart the invader.

While curiosity is part of human nature, it does not justify invading another person’s privacy. It’s especially true if the invader has bad intentions.

Fortunately, there are some simple and foolproof methods that you can use to detect unauthorized entries.

It’s important to note that in hindsight, without any prior preparation, chances are pretty low that you’ll ever find out if anyone opened your door while you were away (unless you have access to professional detective tools).

So preparation is a must. Let’s see how you can make sure your private space remains untouched.

1. Use Ordinary Objects to Check Whether Your Door Has Been Opened

The idea behind this method is pretty simple:

When you’re leaving, you place one or several small objects on the floor right in front of the door in such a way that when someone opens the door, the objects will be moved out of their original position.


You can use any ordinary object that’s high enough to reach the bottom of the door when placed on the floor.

Here are some examples: caps, tennis balls, candies, pieces of cardboard, etc. You must choose an object that you can slide or roll on the floor.

When you’re leaving, first you need to leave the door ajar so that you can reach in with your arm. Next, place the object or objects in front of the door.

Small objects in front of a doorPin

I suggest that instead of placing a single object on the floor, use several items so that if someone opens the door they won’t be able to restore the original layout as they won’t know the extent to which the objects were pushed away.

If you use a single object, that should be as close to the door as possible, however, if you use multiple items, make sure you place some of those a little bit further away from the door.

Also, if you’re using several objects, I highly recommend that you take a photo of the original arrangement so that you can easily tell if there’s a difference when you return home.

Don’t forget that you shouldn’t push the door wide open until you check the position of the objects. You can either use a mirror or reach in with your phone in your hand and take a photo.

Obviously, this method works best with inward opening doors. While nothing stops you from using it with an outswing door, the objects you place in front of the door will be very easy to spot. If your door opens outwards, I recommend that you try the next method instead.

2. Transparent Adhesive Tape: Clever and Almost Undetectable

This one is probably my favorite method to find out if someone has opened a door or entered a room. It’s so simple and elegant.

Let’s see how it works.

All you need is invisible stationery tape and a pair of scissors.

  • First, cut tiny strips of the adhesive tape that are about 1/16 inches wide and 1/2 inches long. You’ll need 3-4 pieces altogether.
  • Next, close the door and stick the invisible tape strips where the door meets the jamb when you close the door (from the outside, of course). About 2/3 of the strip should be on the door and 1/3 on the door jamb. Stick strips at both the bottom and the top part of the door. Double-check if the strips stay in place.

Transparent adhesive tape on door and jambPin

When you return home, all you have to check is whether all of the adhesive tape strips are intact. If the strips have come off the door or the jamb, the door has probably been opened.

Broken seal between door and jambPin

So these little strips work as a seal. If the seal is broken, you know that an unauthorized entry has occurred.

The main advantage of this method is that it’s very simple and quick to implement, costs almost nothing, and works with both inward and outward opening doors (so you can use it on your bedroom door as well as your entry door).

The drawback is that if the intruder is very suspicious and inspects the door thoroughly, they might spot the tapes.

Sometimes it’s better to secure or reinforce your door instead of guessing if unwanted visitors entered your room. Here’s how you can lock your bedroom door from the outside. If you need to lock a door without a lock, make sure you check out these simple methods.

3. Use a Piece of Cardboard, a Matchstick, or a Toothpick

This is another simple way of detecting if a door has been opened previously without using a camera or any other high-tech stuff.

Here’s what you need to do to know if someone has been in your room:

While you’re closing the door (when you’re leaving), slide a piece of cardboard, or half of a matchstick or a toothpick (sometimes a thin twig will also do) between the door and the doorframe on the hinge side (watch your fingers!). Make sure it stays in place when the door is closed!

Here’s how you should do it:

Two fingers holding and placing a blue cardboard between a door and door framePin

Now measure the distance between the floor and the matchstick, toothpick, or whatever you use (for the sake of simplicity, let’s call it an indicator). As an alternative, you can take a photo of it.

If someone opens the door, the indicator will fall to the floor.

The intruder may notice the indicator on the floor but will have no idea about its original place.

So even if they try to put the indicator back between the door and the jamb, you’ll know that someone has opened the door as the indicator-to-floor distance will be different.

It’s an old trick but in most cases, it works reliably.

Important: certain types of indicators (such as wood or metal) may damage the staining or leave a mark on the door or the door frame. If that is a concern, use paper, cardboard, or any other type of soft material, such as fabric or leather instead.

Pro Tip: You can take this method to the next level by using not one but several indicators at different distances from the ground.

4. Install a Door Sensor: a Sophisticated Way of Checking Whether Someone Has Opened Your Door

The DIY methods that we have discussed so far provide a simple and cheap way to detect if a door has been opened and they all work great when implemented the right way.

However, they only alert you of the intrusion subsequently. They do not offer real-time monitoring.

And this is where door sensors come in.

Although there are many kinds of door sensors on the market, here we’re talking about those that are capable of sending you an alert on your phone each time a person opens your door (so it’s not a loud door alarm).

The major advantage of a door sensor is that it allows you to check the status of any door in real-time regardless of your actual location.

SEHOMY 4 Pack Smart Door Sensor Alarms, WiFi Window Sensor

Of course, both the device and your phone need to be connected to the Internet in order to receive notifications.

Most door sensors can be used on both interior and exterior doors, so they can be a great choice if you want to monitor the status of your front door, patio door, or bedroom door too.

Also, most of them are compatible with Alexa and Google Assistant. Regarding the price, you can easily get one (or a whole kit) for under 80 bucks nowadays.

Unfortunately, a door sensor won’t prevent unauthorized entries. So if you really want to reinforce your doors and improve home security, you’ll need some additional steps. You’ll find some helpful tips to secure your bedroom door in this article. And if you need to reinforce your front door, check out this guide.

5. Get Your Unwanted Visitor to Leave a Trace On the Knob or Lever

It’s another pretty neat trick to find out if someone has been in your room or house (or apartment) while you were out (as a side note, I’ve created a complete guide about how to tell if someone has been in your house).

The idea behind this method is that if someone enters your room or home, they will almost always touch the doorknob (it doesn’t matter whether they’re wearing gloves or not).

All you have to do is mark the knob (or lever) so that anyone who touches it leaves a trace.

So how can you do that?

You can sprinkle some dust, glitter, or baby powder (talcum powder) on the knob or just throw some spider web onto it. Of course, you can use any other material if you can spread it over the surface inconspicuously.

Here I sprinkled some dust on a lever:

A close-up photo of a door lever with dust on itPin

You can use a very little amount of oil (or grease) if the material you use won’t stay on the knob (but in most cases, I would recommend against doing so).

The key is that you shouldn’t overdo it, otherwise it may arouse suspicion.

Finally, take a photo of the knob so that you can compare the before and after conditions.

If someone has been in your room, they will unintentionally remove most of the markings from the knob or lever, thus leaving a pretty straightforward trace.

Here someone has clearly touched the lever:

Traces on a door handlePin

I really like this method as it’s a simple yet highly effective way of setting a trap and knowing if someone has entered your room while you were away.

Pro Tip: In the case of a bedroom door, you can mark both the interior and exterior knobs to make the method even more reliable. In the case of an exterior door, mark only the interior knob (as wind and other natural elements may interfere with the markings outdoors).

6. Monitor Your Door With a Motion Activated Nanny Cam

Last but not least, you can hide a motion-activated cam in your room or home to know if there’s any cause for concern.

A nanny cam can be a reasonable choice when you want to know exactly who has entered your private space.

A recording of the entry will provide crystal clear evidence and you’ll know for sure if someone has invaded your privacy.

FYI – In certain situations and countries, installing a hidden cam and making a recording of a person without their consent may be illegal. Make sure you’re familiar with the regulations and laws before you place a hidden cam in your home.

If your goal is to deter your flatmate, you can install a real or a dummy security camera in your room instead of using a nanny cam.

Putting It All Together

As you can see, you have many options if you suspect that someone invades your privacy by entering your private space without your consent.

You can easily take the guesswork out of the situation if you implement one of the above-discussed methods.

Most of them cost almost nothing and require very little preparation. To improve effectiveness, I recommend that you combine at least two of them with each other.

If you need a professional solution get a door sensor or a nanny cam if that’s legal where you live. As an alternative, you can install a new ANSI grade 3 door lock or a security system that will definitely help you keep intruders out of your home.

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About the Author

James Mora is the founder of DailyHomeSafety. He is a home improvement expert, contractor, avid DIYer, and security manager. He is passionate about home repairs, remodeling, and teaching. Read More

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