A mean old man showed me this.
Before leaving the area you intend to secure, find two pieces of scotch tape and a hair preferably 3" long or greater.
Close and lock the door as usual, then tape each end of the human hair next to the door crack near the knob. You want the section across the crack to remain clear of tape, so the finished product should look like an invisible "H".
The tape running vertically and hair horizontally
Then when some creep actually opens it while you are out, you will know and if you measure the hair it's easy to see if the trap was replaced.