Sometimes when nature calls, you gotta go...pull a prank.

And this is a mean one!

After you relieve yourself in the bathroom, carefully unfold one of their guest towels. Wipe yourself with the towel and carefully refold the towel and put it back on the rack. Say the middle of the pile.

Quantity is one direction to go, but it is more obvious. A nice, light racing strip will be less noticeable and might not be discovered for a far longer period of time.