Can you show us a picture of atleast the door and the area around? if possible a picture of the room too?? A good idea is wireless cams for like $20. They're of very poor quality, but if it works for you good then. Maybe pics of the place can give us a few nasty ideas...
Heres another inexpensive idea.. I don't know how possible it is though..
You know in schools how if you pull a fire alarm it leaves a paint mark or something on your hands... you could use that.. and pray that the person doing some "bing bong" on your door isnt smart enough to wear gloves..
