That would make me angry as well. Probably fell out of their mouth or they sneezed and were too embarrased to get down and actually look for it.

I used to have a family that would arrive to pick up their son with Tim Horton cups in hand. A couple of times they actually left their empty cups on my stone bench just outside my door. I too was livid! When I confronted them, they denied it. But they were the only family that picked up with Tim Horton's cups in hand!

I termed them shortly after for completely different reasons, but surprise, surprise....no more empty Tim Horton cups on my bench after that.