New beekeeper. took delivery of bees on april 5 and things went great. population grew dramatically, brood box has comb on every frame and the bees seemed to be overflowing with bees hanging out on the outside of the hive. Added the flow super around May 27. Bees immediately started working in the flow frames and seemed happy. Yesterday noticed that the flow super was completely empty. It seems all the bees that had populated the flow super are gone. The population in the brood box is still good, but its down dramatically from May 27.
Are you inspecting the brood box every week? You should be looking for queen cells weekly during the nectar flow season, and be prepared to intervene if you find them. Otherwise, if you let them swarm, you will lose most of your colony and a lot of your honey.
Do most people in your region use 2 brood boxes on their traditional hives? Most regions in the US do. The Flow hive was developed in a subtropical climate, where they can get away with one brood box. However, it is really just a different method for extracting honey. Everything else about beekeeping in a Flow hive is the same as traditional hives. Traditional beekeepers in my region use 2 brood boxes, so I do too. If you don’t, you may have trouble getting your colony through winter.
Len - have a really good look in the trees and bushes around the hive. There is a chance the swarm is still nearby, but they can be easy to walk right past.