Thanks to all who helped me id the bird which was here so briefly. Based on consensus and studying Sibley , I do think it was a Black Throated Gray Warbler. It would be a first in our garden, though who knows if one slipped through. I seem to recall a spot of yellow above the eye, which I think made me think of the Townsend's since the mask looked so similar.
I've had many Townsends so this was a nice new addition to my yard list.
Also, while watering today, found a dead crow beneath some plants.