I wish I could be of help, but I have never heard of such a thing happening in a container like that before. I would say that I doubt there is any reason for panic. My cats catch flies in the windows and eat them whenever they are available! They are fine. It's always a good idea to ask the vet just in case they ingested fly eggs but I would think all will be fine until you can contact them. My guess is...we wouldn't want to know what they may have gotten into and eaten at some point and time!
Good luck and I'm sure everything will be ok.