Missing, Presumed dead

So I went to visit one of my apiaries today and noticed one of the hives with no activity at the entrance. On opening her up there was nothing inside; no brood, stored pollen or honey. All that was left was half a handful of dead bees in the bottom and a whole lot of powered cappings on the floor. Now there were a lot of wasps around, but this hive had had the smallest of the three entrances of the hives there and the one with the front door wide open was thriving in spite of the wasps.

So what had happened? Robbed and killed off by wasps? Marie celeste style absconding? Colony collapse disorder? Mass pesticide-induced bee loss by losing their way while out foraging?

I guess I will never know; but I will surely freeze the frames before using them again incase it was disease related. As there were next to no bodies I doubt it…