If their scanning system/cameras are anything like what we have here where I live, if you do not have a toll tag displayed or if it doesn't work for whatever reason, your vehicle tag has a picture taken of it. They then search their data base for that tag number and if you have an active account tied to that tag number, they charge your toll account. That could explain why you never got a warning in the mail.
Good point. The state should be able to verify anyone's travel at that time that day even if the transponder was broken, because on the other side, if there was no active account, the same cameras had photographed the plate, which is how they catch scofflaws. So, either way, there should be photographic and/or electronic evidence of MP and our eyewitness traveling that road that day and at that time.