Likely: Scan everyone's home while en-route to the 911 call with an infrared camera. Or scan all of the license plates and faces of people along the way.
US law is more murky if they are responding to a perceived emergency. That can give probable cause. Imagine someone calls 911 due to smoke coming out of the neighbor's house. Clearly the drone can legally use IR to look for fire in the house. But it gets complicated fast, which is what worries me. Now imagine a car backfires, a microphone array reports it as a gunshot, and a drone shows up and starts scanning the nearest apartment building.
Possible: Perhaps crash into someone? Or worse.