How can we tell if someone has been in a place. I'll list my thoughts really quickly both natural signs and indicator (traps).

natural Signs
-Spider Webs
-foliage damage
-dust
-foot prints (grease oil etc)
-garbage
-spit
-excrement
-scents
-heat left by from a person (quickly dispiates)

Indicating Devices
-Strings (on cabinets, doors)
-Trip lines
-Pyrotechnics
-Krill Lights
-Chemicals
-Spy Dust
-Tape (sticky side up)
-Forensics

A hair across a lock is something that i've heard of used to determine if a place is being opened up or not. Put a hair across the keyhole (can't remember what they used to keep the hair from blowing away/falling off, but the point of it was, a hair across the keyhole will be broken/displaced if anyone pute a key in, showing that someone was there...

Posted by Mark Indicating Devices -Strings (on cabinets, doors) -Trip lines -Pyrotechnics -Krill Lights -Chemicals -Spy Dust -Tape (sticky side up) -Forensics

moe detail about those would be very interresting

Posted by Jester A hair across a lock is something that i've heard of used to determine if a place is being opened up or not. Put a hair across the keyhole (can't remember what they used to keep the hair from blowing away/falling off, but the point of it was, a hair across the keyhole will be broken/displaced if anyone pute a key in, showing that someone was there...

You would probably be able to affix the hair across the keyhole by using clear nail polish. You'll only need a tiny section of hair (less than an inch) and the polish will be pretty much invisible. Also simple enough to carry - small bottle of nail polish in your pocket, and you can get the hair off your own head.

The same technique can be used on plain old doors, cabinets, drawers, anything that moves. The hair is thin and weak enough that breaking it won't take any force and thus the intruder won't feel it.

Posted by Jester(can't remember what they used to keep the hair from blowing away/falling off

Spit does the trick. Anything stickier and you'll get some noticable resistance when the key touches the lock.

What does the sticky side up tape do?

You get grit from boots etc. In small patches it can tell you lots.

Placing a strand of hair above the inside of a door is what I've done to know if my roommates have been in my room.

1. Find someone with long hair.

2. Take (or ask) for a strand.

3. Use a bead of hot glue to affix the end of the strand above the inside of the door.

4. When leaving the room, be sure you close the door so that the hair is inside, and not caught and trapped outside by the door.

If someone has been in the room, they will likely not notice the strand, and when they close the door upon leaving the strand will be trapped outside.

Good luck.

I've used these and learned a lot of them from Sherlock Holmes stories. And the Discovery channel of course

Uneven levels of dust really has to be the best indicator for determining how long a place has been empty, especially on door knobs, cabinets, and the area the door sweeps when it opens (for some of them).

And noise, the best indicator but the quickest to decay. Always worth a check.

Hair+spit works for short term, you'll need glue/polish for long term.

and an alternative to tape is gluesticks - it's less obvious, but more of a mess.