Great question, and it looks like you did well to intervene and resolve.  As the weather warms, it is quite likely that there will be hungry colonies looking for easy food sources. Basically with climate change, bees stay active later than food resources are available in the landscape so they eat down their winter stores, and and also become active in late winter before many flower and pollen resources are available. You may have neighbor beekeepers who are not feeding their hungry colonies, or hungry feral colonies nearby (the Labor Day fires smoke could well have messed up the poor feral colonies so they ate down their winter food supply). I would definitely keep and eye on that colony. If it is strong and you have made the entrance one bee wide, they should be able to defend. If the robbing continues, you might possibly consider offering an alternate food source well away from your vulnerable colony.