Remember the conversation Agent Smith has with Morpheus in The Matrix? The Virus speech. I have to admit that he had a good point even if he was a psychotic computer program who enjoyed his job far too much. We humans can be really disrespectful to our surroundings and other humans.
Nothing like a walk down by the river especially if you have a good drink with you. Maybe it was a first date or a walk back home after a night on the town. Whoever it was, they left their calling card. Two shakes left on a step… for someone else to tidy.