I knew there must be a nest nearby, and sure enough I quickly found one low in a young Hemlock. I also quickly noticed though that a large Eastern Garter Snake was draped over the branches just above the nest!
After watching for a while with nothing happening except the continued agitated chipping of the adult warblers, I moved closer and caught the snake. I couldn't feel any lumps of eggs or nestlings in its body, but given the large size of the snake this was not too surprising. The nest was empty:
I have no doubt that the snake had just emptied the nest of the eggs or nestlings. Nest predation by garter snakes is rarely observed, but has been documented several times in the past.