The bees are fanning, therefore thy are NOT robbing, there is a colony in there. Proably colonised by a swarm that happened when you were not at home. Opening "to check" will chill them badly, maybe kill them - it will be small as you have not seen it before. Opening breaks seals, cutting brace comb removes their draught excluders. Do not open, do not treat (another stress). If they make it through winter they will probably thrive next year, very possibly without treatment if they have made it this far.