Disappeared

I put my second dose of varroa treatments on, and fed every hive today (using fondant only). However when I lifted the roof of my Rose/Warre hive it was empty! No bees, no brood, just clean empty comb. There were bees and brood a fortnight ago when I put the first dose of varroa treatment on. I cannot explain why and have spent the rest of the day theorising.
Tonight I went to a local bee talk which was very good - my thanks to Bob Gilbert the speaker.