I spent about four years working at a pub while I was at uni. I have several habits very deeply ingrained, one of them being to clean every surface immediately. (I never notice dirty floors - the cleaners did that!) We also had a list of jobs that had to be done every week, like cleaning under the fridges, the front of the bar, the windows, and the floor behind the bread rack. We were fired on the spot if we missed a spot. (Not quite.)
So one thing I notice is if someone has just mopped around things on the floor.
Yesterday, I was working in my office, and the cleaner was in the courtyard behind me. She was making an amazing ruckus, banging buckets and humming to herself. Despicable! I determined to go and yell at her for making so much noise in an office building. How dare she interrupt us godly solicitors! I earn... well, at least a few dollars an week more than she does. Who does she think she is?
I then noticed that it was well after 7pm, office hours were over, and I had, once again, missed dinner. I slunk home, and as I was leaving, gave the cleaner a cheery nod.
Did I ever mention how good a job they do? And for such terrible money!