Where there's smoke, there's....
My apartment building has a fancy centralized fire detection/security system. One sunday night, I was heating the oven to cook a frozen pizza; when I opened the oven, a puff of smoke rolled out (must have spilled something earlier.) To my annoyance, it set off the smoke detector/alarm. I opened a window, cranked the ventilation fan up, and the alarm turned off again after about thirty seconds, to my relief.
About ten minutes later, I hear a knock at the door. I open it, and lo and behold, it's a cop! Apparently our alarm system passes an alert through to the local police station if nobody is around in the building complex to check it out. Was everything all right, he asked? Why, yes, no problems, just some smoke from the oven. Could he take a quick look around, he asked? Sure, I said (I try to be friendly with police; most are just trying to do their jobs.) As he was about to come in, I remembered something: My apartment wasn't exactly ship-shape at the moment. Specifically, after a rather fun weekend, it was strewn with condoms (not the used ones, of course), bottles of lube, and various sex toys (including a dildo.) I froze and shot an arm up between myself and the door, blocking his path but giving a clear view of the (clean) living room. "Ah...eheh...see, nothing wrong here!" I said with a sheepish grin.
He thought about for a second, nodded, thanked me for my time and left.
I guess the lesson is you never know who might be dropping by, so try to pick up after yourselves.

(And you can change your mind about letting police in/look around; no need to be shy about asserting your rights.)