Okay, I found a nine-second clip on YouTube. Unfortunately, there's not enough context so I'll describe it below:
A drone carrying a motion-sensing grenade has just entered through a window and is landing in the center of the living area in 221B Baker street. Sherlock, Mycroft and Watson are backing away from the center, like vertices on an expanding triangle. The conversation continues:
Mycroft: "The motion sensor has activated. If any of us move, the grenade will detonate."
Sherlock: "How powerful?"
Mycroft: "It will certainly destroy this flat and kill anyone in it. Assuming walls of reasonable strength, your neighbors should be safe. But as it's landed on the floor I am moved to wonder if the cafe below is open."
Sherlock: "It's Sunday morning, so it's closed."
Watson: "What about Mrs. Hudson?"
[Camera pans down a cutaway to 221A where...]