Absolute diagnostic. I was going to ask if @deemann1 did that.To check for varroosis you hold the frame up so that sunlight hits the bottom of the cells. If you see white deposits inside the brood cells, this is varroa poop. If there is a lot of excrement then varroa is most probably the culprit.
Probably too large a space to keep warm, why leave a super ?A pal been to his apiary. One decent sized colony, topped up with fondant, and a super of stores has decamped..not one bee remained!.. In November!...they had room, so any thoughts why? I know of starvation swarm but these had no excuse!