Compton Stealth Shelf
As your sexual partner goes to the bathroom to "freshen up", you scout their room and find an object to place up your rectum before participating in intercourse. After the act, as they go to the bathroom to clean up, you remove it and place it back without them noticing.
“Hey I had a great time last night! But did you happen to Compton Stealth Shelf my jumbo sharpie?”