Varroa

 I have had a few beekeepers tell me they went and checked their bees. They looked great two weeks ago and now there were no bees in the hive and all of the honey was robbed out. There were no dead bees in the hive.
 All of these absconding colonies most likely had high levels of Varroa. When Varroa levels get high, the bees will drift away. Leaving no bees and unprotected honey.
 If there were piles of dead bees one could say they were overcome by robbers and the colony was killed in the robbing melee.
 Failure to treat for mites leads to absconding or a weakened colony that will not survive the winter.