I also remember reading about this in a Miss Manners book from many years ago. It's a dumb thing to do, unless you have a history of mutual practical jokes.
Our guest bathroom medicine cabinet has Tylenol, antacids, Benadryl, tampons, and a few other things that a guest might need but not have. We don't keep anything private in it. All the prescriptions, etc. are up in our bathroom off of our bedroom. One time, a guest who was staying over with two other guests asked if he could use our bathroom. I told him it was fine. I was surprised when he took a shower. "Use the bathroom" usually means the toilet and sink. I was embarrassed because we hadn't specially cleaned our shower, or set out towels, and he went hunting around for a towel and washcloth.