Update: we got home last night at 7:40 p.m. to a letter on the door and voice mail saying the plumber would be in, and a note on the door saying the drain would be disconnected at 8:00 a.m. We were up all night making sure the plumbers could get to what they needed to... and just now I called the condo manager. Oh... they were waiting for me to call. (Funny, there wasn't anything about that in the letter.)
Current situation: we now know where the water's coming from, and it's not the sink. The sink is bone dry. The water is coming out on the floor, apparently out of the wall. The floor was dry, until the water started up again just now.
Condo manager called back; the plumbers will be in on Monday. When told water is pouring in again, he said the maintenance guy for the complex will come by. I hope it's soon.