The drain doesn't appear completely drained at this time. Not sure if it was draining slowly when this issue made itself obvious. I suppose it doesn't hurt to check at this point.

I don't have access from below. There's a drywall ceiling in the downstairs bathroom, then the subfloor. One of the cast iron pipes nearby has a point that is terminated with a cap that can be unscrewed, but I'm doubtful I could apply enough leverage in the limited space to open it.