I observed this very strange event: One user of a shared database updated the database when prompted upon opening. The database did not appear open, but on investigation, the database had opened without windows. But the database had the wrong file name. The name of the file was TimeClock, but when opened, it was the database Timekeepers, which is normally opens without windows. I did not carefully document the steps leading up to this, but will be alert in case it recurs.
I discarded both databases, then downloaded both Timekeepers and TimeClock again and the problem was resolved.