A word for evidence used to tell if someone has been in your room

Question

I know there's an actual word for this. I used to know the word, but I've lost it.

The word describes a category of methods that someone uses to detect if someone has been in a room, or opened a door, or drawer, or folder, etc...

Examples: 1) Putting a strip of paper in a door jam, so it falls if someone opens the door. 2) Putting powder on the floor that will be removed or scattered. 3) Putting a ball behind a door in a specific location, so if the door is opened, it can't be placed back where it was with certainty. 4) Putting tape across a drawer that will be broken if the drawer is opened.

I imagine this word would be associated with forensic science, security, or criminal / legal work.

Answer 1

In the British Army, that's just called a 'tell'.

Answer 2

You may be thinking of:

telltale:

2. Something that indicates or reveals information; a sign.¹

Also from Wordnik

n. A machine or contrivance for indicating or recording something, particularly for keeping a check upon employees (factory hands, watchmen, drivers, etc.) by revealing to their employers what they have done or omitted.²

n. Something that serves to reveal something else³

In this way, the paper you place in the door is a contrivance for indicating or recording / revealing whether a person has entered the room.

And as @SamBurns correctly states, this is often shortened to simply a tell.

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^{1 _{American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.}
2 _{Wiktionary, Creative Commons Attribution/Share-Alike License}
3 _{The GNU version of the Collaborative International Dictionary of English}}

Answer 3

The "category of methods" you list in your question ("has been in a room" or, opened a door, drawer or folder) all fall under the umbrella term "intrusion". So maybe the term you're looking for is:

Intrusion Detection System

It's a term commonly used in computer networking to describe security applications that patrol for malicious activity (Wikipedia).

Maybe the term has other applications? Home Intrusion Detection System, perhaps?

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Also, your example scenarios, when they work as planned, may result in a:

giveaway

a thing that makes an inadvertent revelation ~ Google definition

dead giveaway

an unmistakable and definitive clue ~ Dictionary.com

something that reveals a fact or an intention completely ~ The Free Dictionary