If its winter, in the return duct for the central heat
spring, on the front door mat
summer, in the lettuce crisper drawer of the fridge
fall, inside a jack o lantern
In the toilet and flush with the exhaust fan running, you dirty heathens. And remember to lower the seat lid then thoroughly wash your hands before leaving.
If there isn't an exhaust fan, I personally run the water in the sink so as to cover up some of the sound of plopping.