Ceiling fan dripping in bathroom
Turns out this was not a plumbing issue, even though all signs pointed to it: related to bathroom use (even though with long delay), traces of moisture in hardwood floors tracking from upstairs bathroom wall to leak, 'bathroom' smell.

In fact someone installed the drip pan under the AC in the attic at an angle, so condensate leaked, soaked the insulation, tracked along second floor ceiling to return air register, along that vent down the wall onto the subfloor... There it found a convenient knot, dripped on straps, and found its way to the fan.

Fortunately we haven't used the AC too much this year, so the joists are fine and hopefully the hardwood floors will dry, too.

Lesson learned: you may want to think x is your problem, when really it's y.