Some details about versions would be handy.
Firstly, sharing FMP files from a network server is not good practice.
Secondly, make sure the auto-login is disabled in the database's File Options. It's probably opening with the default Admin account.
Your second comment: "auto-login" was Spot On.
The file is shared with my users via a FileMaker Hosting Service. Knowing that, is it still "Not good practice?"
A FileMaker Hosting Service is good practice!
Glad the problem was sorted reasonably easily. :D