I thought first it might be the one in the dining room right outside Misha’s room, but that was a smoke alarm and carbon dioxide monitor. I had to quietly go to the basement to retrieve a step ladder.
Misha must have heard me anyway and opened her door (with noise cancelling earplugs still in), and I discovered the chirping was from the alarm right inside her bedroom door.
She helped me change the battery and finally there was silence. I think it had been chirping every 30 seconds since 2 am.