Update: found water pooling at the base of my new vanity, traced it back to the p-trap
Last Saturday I finally finished tiling and set the vanity in my upstairs hall bath, and by Sunday morning there was a ring of water soaking into the grout line. I pulled the cabinet back out, dried everything with a box fan for about 6 hours, and noticed the p-trap slip nut was only hand tight and weeping on every flush. Swapped the plastic washer for a rubber one from the hardware store, snugged it with channel locks, and ran the sink 20 times with a paper towel under it, bone dry. Did I get lucky that it was just a loose nut and not a cracked drain arm, or should I be worried the drywall behind it soaked up moisture?
Why worry now if it's bone dry? The washer swap was the fix, that's normal stuff. Just keep an eye on the ceiling below for a few days, if it stays dry you're fine.