Well, since one of mine didn't involve keys, I might as well put it here....
We were having a meeting of 20-30 people in a small office and someone had 3d-printed a heart gear (
one of these....)
and we were passing it around, marveling at how smooth and bloody neat it was.
Someone next to me was rotating the gears and a larger one popped off. It hit the floor (we all heard it) and we all
immediately looked. We looked behind us, backed up our chairs, looked again...nothing.
By sound, I estimate that it landed pretty close to me (within 5 feet, probably under the tables), but it was nowhere to be found. The area under the tables was pretty clear. The heart pieces were bright red (as in the link), the floor was mottled white tile.
After the meeting several of us got down on all fours and scanned everywhere. Apart from the tables, there was nothing it could have rolled under.
Fast forward a year later and we moved out of that building. I, personally, was one of the ones to clear out that office and it never did show itself. I'm still waiting to be poking around in the ceiling of our new place and have it randomly drop out of the rafters....
Yes, I considered that someone might have nicked it, but I seriously doubt anyone in that group would have done so; also, when I say we immediately looked, I mean it. Promptly, right that second. I doubt anyone would have had time to bend down and grab it.