Bees gone missing ???

I was visiting my father-in-law this week and we went to check on his bees in the out-apiary. He has 3 colonies there together with another beek who has 2.
We opened 2 hives and found many bees in there, very strong and healthy looking. Then we opened the 3rd one and we were faced with a life-less hive :(

No bees between the frames. No buzzing, no life :(

He treated these hives in september so he is sure they were very strong and looking good, lots of brood and lots of stores as well as pollen. He is puzzled. We opened the hives of his fellow beekeeper too and found that one of his hives is also empty of bees! Lots of pollen, honey but no bees at all!

There are no dead bees nor chewed wax on the bottom board which would indicate robbing or even disease. Its most defently no CCD because in that case there would still be some dead bees in there or even a Queen with a small amount of house bees.

I checked the brood comb and found many small white spots, which are Varroa feces, in the cells indicating high Varroa load. There are also a few capped brood cells which I have checked for AFB but that was not the case. My father-in-law thinks they have absconded because the Varroa load was too great. This is the only colony he never culled drones from (integrated pest management). I told him that I never cull drones and for the last 3 years I did not have such issues.

The hive had plenty of stores. Not sure why the ladies left so much food just to swarm into certain death? We also considered that someone stole these colonies but after inspecting the frames we quickly realized that is out of question because all frames were propolized and braced with wax. If someone did steal them the propolis and wax would have been broken. One of his hives was stolen from this place last summer which was the reason we suspected someone stealing them.

We are both very much puzzled with this mysterious bee disappearance 

On the positive side I am soon turning 40 and he gave me one of his traditional Danish bee hives as a birthday present :) sweet!

This horizontal hive is traditional in Scandinavia and is called Trugstade in Danish and Trågkupa in Swedish. It is a very well insulated hive.