Someone came out to the main entrance. Barty saw their outline, but that was enough to make him watch, wait a little longer for signs of anyone else. This person moved smoothly, quietly. This was no teenager wanting a prank, and it didn't have the feel of robbery. A thief was more likely to case a place, and this person had not been able to. Or precious light, a dead give away, would not have been wasted to read a sign. This person was searching for something in the dark, which meant he couldn't search any other time. Outsider. And alone Barty judged.
Silently, he cast a cutting spell, aiming for the legs, and Barty swiftly moved to a location with more maneuverability. His presence was declared.