The place I worked at had a long loading dock. I noted my boss would get the brush out to sweep off water that had formed in a low/worn out section of the dock.
I took to topping up his 'puddle' every day, he swore blind after a few months that he was going to get in maintenance to dig up the part of the dock, as he had checked the down pipes and roof, and the water could only be 'seeping up'.
By this time I had been doing it for 5/6 months, so I filled the puddle right up, popped a couple of yellow duckies on it, and took a few photographs, and didn't do it again.
I popped the pics in his birthday card the following month, his face was a picture when the penny dropped.