A penny for your thoughts...

[Patrick Sweeney]

There's the old-school spy trick of putting a small bit of paper or a splinter of a matchstick in a doorframe as you close it, but not where it is obviously visible. When the person snooping opens the door, the object falls. Properly placed by you, they don't see it fall. When you go to check, if the whatever is on the floor, someone went through that door.

Too bad to hear of his passing.