Re: Not sure what happened but bees are gone!
A post mortem is hard to do after the scavengers arrive even when you can see the hive. I would concentrate on saving the live hives first.
Dead bees out front, nobody home, robbing most likely. That may be just a result of problem not the cause.
Did your queens make the move alive? When did you last see brood in live hives? Beetles? Ants?
You need to know your live hives are good first. What you find there will probably tell you what happened to the deadouts.
4 yrs, 2 singles, 7 nucs, T lite; godfather to brother's 3.