I heard noise early today which was probably the branch being partly dismantled, and the leaves from the part overhanging the near side of the roof ended up on the roof itself. The remainder of the fallen branch is now on the ground, sticking out from behind the building, and it looks as though in the process of taking it the rest of the way down it damaged the fence on the side of their yard. With the branch now out of the way I can see the broken spot on the tree itself. Well, two broken spots, actually. It looks as though one branch fell and hit a lower branch, and that took the lower branch off as well.
By the time I got outside (I merely went back to sleep during the very early noise) nobody was around, but I expect they will come back later today to finish the job. I was surprised that the branches that fell were as small as they were. If one of the leaders (the really big branches that grow directly from the trunk) had fallen instead, it would probably have crushed the ridge of the roof on that workshop, and I would have seen the damage yesterday. But the larger of the two fallen branches is probably only about thirty feet long, and has lesser branches that probably make the whole assembly only about ten feet across. The smaller branch, which was the higher one that must have broken first, is not visible. It must be on the ground on the far side of the building.
My guess would be that this is another case of a section of tree having been hollowed out by nesting carpenter ants. If so, one or more the the tree's leaders could also be infested, and could still come down. The part of the tree still standing will be inspected, of course, and any parts that are infested will be cut down soon. That could be a noisy, day-long process. I'm just glad that this time no utility lines were in the way. When the tree at the end of my block lost a leader a few years ago we were without power until the next day, and I really don't want to be without power during a heat wave.
Ah, the perils of living in a former forest.