Someone missed the hall litterbox, and left a puddle between the box and wall. We think.
We're not sure because a local landscaper came through and did his best to cover the problem. He moved an inch high dune of litter from inside the litterbox to between the box and the wall.
When I found this pawwork, I had to stop and admire the effort before cleaning it up.
(There was a tiny puddle left under the edge of the litterbox where it had seeped. But the landscaper had gotten everywhere he could reach.)