[beat!the!champ!]

If you're just trying to find out if someone entered the room, you could try this if it's possible:

- get a set of dice, or those toy blocks with letters on them, anything similar. More inconspicuous, the better. A set of coins with different years on them maybe.

- stack them up inside the room close to where the door swings so that when the door opens, it'll knock over the blocks or dice or whatever

- be sure to set the blocks or dice so that they spell out something you'll remember, such that when they're knocked over, the person won't know how to set them back up.

- leave the room through the window or something. Or set your blocks up on a piece of paper, and pull that paper under the closed door or something.

I'm assuming there's only one exit/entrance though. And that the door opens into the room, as opposed to outward. If it swings out, you could do something with string on the doorknob attached to a piece of paper on which your blocks are stacked inside the room.

PS. Oh wait, you already know someone's going in there.