Now that his eyes had adjusted, he could finally see and pick out the outlines of some sort of hole above the bar. Presumably that's where the shot had come from.
He could also see a spreading pool of blood on the ground encircling what remained of Jiraiya's head, which was surprisingly little. Whatever kind of bullet that was, it probably wasn't a handgun. Entrance wounds weren't usually that big, so Trowa would guess shotgun or something similar.
It'd sounded like a large gun of some sort, too, when it hit the ground-- and weren't they lucky that it hadn't gone off again? The fumbling badly enough to drop it in shock after the sound that firing the gun had made, coupled with that "G'way", and Trowa thought he was dealing with an amateur. An amateur who had gotten a headshot on his first try.