The best idea I can come up with is that they swarmed, and the ones that stayed didn't make a queen. Or else they got weak and another hive robbed them out. Some of the comb looks torn. But there's no drone cells or no laying workers. And there's a robbing screen on the front. So I figure the bees that were in there were after the syrup in the top feeder. I had put a screened oil-tray small hive beetle trap underneath the hive, and there were lots of beetles in the oil. I guess the beetles got in there and since there wasn't anything for them to eat, they ended up in the oil tray.
So here's the 10-frame hive now:
There's some perfect comb in the deeps, so I'll stick them in the freezer for a few days and save them for next spring. If I catch a swarm or do a split, that comb will be useful.
I'll probably use the boxes for swarm traps. I think I'm going to go all 8-frame hives. If my back keeps going at the rate it's going, I may go to medium brood boxes and shallow supers.
On a positive note, there's still lots of goldenrod and other flowers blooming, and the bees are bringing in tons of pollen.