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How to Check if Someone's Been in your Room
This is a very simple and nearly foolproof way of finding out whether someone came in through a door in your absence. Better yet, it is completely free and very low-tech. You can use this method likewise on outside doors (whose lock might otherwise be picked and you wouldn't notice), and on inner doors if you want to check whether people are respecting your privacy in your absence.
Step 1: Pick a small piece of paper or cardboard.
Step 2: Fold said piece of paper or cardboard into a thin strip.
Step 3: Use a standardized size you have with you at all times - in my case my waterproof memo book - and use that to determine the height at which the strip of paper is to be wedged between the door and frame. An easy alternative is to use a pen, your glasses, or any other object.
Step 4: Wedge the piece of paper between the door and frame. Try to leave only the tiniest protrusion, that should be as invisible as possible. Hint: Elect a piece of cardboard or paper that is as close in color to the door as possible, to avoid it being seen. You could also use ink to color the paper appropriately.
Here you can see the piece of paper barely visible between the door and its frame.
With such a system, when you return to your house, apartment, or room, you can check whether someone has entered through the door. Unless they've seen the piece of paper, any use of the door will cause the paper to fall out, like so:
Just make sure not to open the door yourself before checking the paper!
Step 3 is there to ensure that if the intruder sees the piece of paper on his way out, and realizes what it is, odds are he will replace it at the wrong height, which you will be able to easily check.
(I learnt this particular trick from a TV show called Jericho, which in my opinion is pretty much on par with Firefly.)