Put some invisible luminous paint that appears when touched on a pair of panties and leave them in an obvious place (bathroom etc) You will soon find the culprit
To me they're knickers as well and they were always knickers to all Australians for years. Suddenly they are not thanks to ....... oh crud - I'm rambling. Um, yeah. I luuurve girls' underwear. Maybe not when it's dirty but I get a kick out of shops like "Bras and Things" where I can be surrounded by girly underwear. I'm also keen to detour when walking through department stores. When staying in backpackers hostels and places like that I can't get enough of doing my washing knowing that I'm probably going to be surrounded by it when hanging my stuff on the washing line. At the last hostel I was staying at the girls liked to hang their underwear on my bed. I certainly had no objections.