Meeting rooms generate empty coffee cups. You'd have to walk into a room immediately the cleaners had just left to find one without them. And half-used bottles of still water too, and often small brown carrier bags from the deli and somebody's security card. That's because of one of the most fundamental rules of the business world is:
An Empty Coffee Cup Has No Owner.
Thus coffee cups will be abandoned with little or no thought for who will clear them up.
Does it matter? Does it matter! Sure. In business-as in Life-the small stuff (abandonment of the dirty cups) is an indicator of the big stuff (management of the project). And the external stuff (state of the room) is an indicator of the internal stuff (state of the thinking).
Take a discrete note of those who own their cup and dispose of it thoughtfully. Better than any expensive, flash assessment centre for sorting out the best future leaders.