I was once taking my newly arrived guests through my apartment and showing them the mini bar when I relised I'd accidentally left a 3/4 full bottle of (cheap) vodka in the fridge, so I said "help yourself to the vodka".
A few days later when they checked out the guest mentioned he had stubbed his toe and wiped up a spot of blood from the floor. He also mentioned they'd put the bedsheets in the washing machine for me. After they'd gone I found blood spots on the floor all around the floor and even in the fridge (!) - and of course when the sheets came out of the wash they had blood stains on them as well. So I put two and two together and relised he'd got trashed on the vodka, tripped and stubbed his toe and hidden the evidence. And what's more, this guy was an Airbnb host himself. Incredibly rude.