One of the leading causes of robbing is HBH. September is "wind down" time in the North. If they swarmed they lose half their bees immediately... Smells are always worth investigating. Sometimes they are due to a change in nectar, but sometimes they are indicative of something seriously wrong like small hive beetles (yeast and orange) or AFB (rotting meat). I would always look for signs of AFB if there were strange smells. Sunken cappings and dead capped larvae that "rope".