It was definitely raccoons using up the water and leaving a dirty sludge in the water bowl. How? They were climbing in the water bowl.

They managed to knock the bowl off the table, so I cleaned it but refilled it on the ground to prevent breakage. The next day the water was still in the bowl. Maybe they only like elevated baths? Or they found someone else’s water dish to bathe in. Or our long-awaited rain shower refilled their usual puddles. The rain was glorious. I sat outside and just watched.