I see it on a regular basis. Likely an accumulation from years of bees getting inside, confused, can't get out and die by the windows.
Actually that makes more sense. I expected to see at least some comb where the respective "swarms" clustered but there was none. However, in both cases 99% of them were on the outside of the curtain not the inside by the window. Either way it was creepy down there, but I've been in worse.