It is amazing how dirty the pans can get over night. I leave the pans fairly clean at night and full of water.
In the morning the water is gone and dirt has dried hard in the pan. It won't come out with just rinsing.
Since these pans are outside the chicken pen I think the local wildlife come by at night and make themselves at home.
Some of the animals probably just want a drink, but I'll blame the raccoons for getting the pans all messy. They like to wash their food in water.
Sometimes the pans are knocked over, but most of the time the pans are just empty. Except for the dirt.