Workbook Not Shared, Still Opens?

[Jeanette L]

We have one user on our LAN with a Laptop and docking
stations. For some reason, when he opens certain Excel
spreadsheets, other users can still open them without
getting a warning message that another user already has it
open. Other spreadsheets work as expected (the second
user gets the warning message).

I don't see any obvious differences in the worksheet
properties or Tools (these are Not Shared workbooks).

Any idea why this is happening? What should we check?

Thanks.