>>Are you sure that the bees aren't just using the hole in the cinder block to access a nest that's actually built in the ceiling or roof itself?

Yes, this a typical warehouse building in this area.  Steel construction with cinder block walls.  The nests at the roof line with entrances between the top I-beam that supports the roof and the top course of the cinder blocks, right over the loading docks.  Inside the cinder block walls are bare and and the I-beam is exposed.  No sign of bees inside the building.

The ceiling inside is the standard corrugated/wave sheet metal construction that is bare from inside.  On top is a flat, black tar roof that gets extremely hot in the summer.

I've head a couple of cases where bees would go through the cinder block and right into a 2-4 stud wall but unfortunately this is not the case here.  I have never taken colonies out of cinder blocks and frankly was surprised the bees found the narrow channels sufficiently large for a nest cavity [& overwintered successfully!].

Waldemar 

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