My husband got out of the car to see what the problem was. When he had no luck, my father in law followed. The front door had several old locks on it. We figured out that one of those old locks had caught, one that we didn't have a key to. My mother in law admitted she had been messing with that old lock earlier.
Fine, we thought, we'll go open the back door. But our keys for the front door didn't match the lock on the back door. We hadn't thought to check that before. We went back to the front door and tried reaching through the mail slot, but no luck. It was made to keep people from doing that. We were locked out. The darn house had locked us out.
The back door was just a handle lock, not a deadbolt, so with a bit of work, my father in law was able to pick it (Once again, we were very lucky to have him with us), and get us back into the house. An hour after we intended to leave, we headed out to Silver Spring for dinner, making sure the door was unlockable as we left. My father in law removed the strike plate that evening, so that lock will not lock us out of our own house again.
The lock that locked us out, now disabled