I printed out a suggestive ad for a different sort of event, faked a handwritten note on it between the hosts, and left it in the bathroom. And then waited to see who would read and comment.
My assumption is that people do snoop, and they would love to comment, but no one wants to be first.
I started off with the paper folded, under a small glass vase - but still with a cheeky view. No comments. Then, after a few more "users" visits, moved the vase. Nothing. Then unfolded in plain view. I was ready to tape it to the mirror next, but the hostess saw it there and brought it out. Laughs all around, people had noticed it, but reluctant to comment of course. Helps if the hosts are good sports (they were, and threw a great party).
I like the suggestion noted here about party snoopers, stuff a bunch of marbles in the medicine cabinet and wait for the trap to be sprung.
BTW - I think we will be going to MLO November 12.