No sign of a drip from the ceiling, which would've been pretty odd. The coat closet at the top of the stairs is kind of above that, but a few feet away.
I headed to the sink to dump out the liquid, whatever it was, and it was slightly brown, which made me nervous. I dipped my finger in it, smelled it... just a faint leather smell.
I put on a different pair of Birks and took the dogs for a walk, and it wasn't until I got back and hung up Penny and Carter's leashes that I figured out what had happened.
Penny's leash was still slightly wet from yesterday, when I gave her a quick shower after she rolled in something nasty during our afternoon walk. I guess I had hung up the leash right after that shower, before going out to the Bet the Farm Wine Dinner, and it must've been right over just that one Birkenstock.
I had created the puddle. Mystery solved. It should be dry in time for me to wear that pair of Birks tomorrow.