Mapped drives appear mysteriously.

15 PCs Running WinXP-SP2 on network with file server running Server 2003.
All PCs have S:\ mapped to the shared drive.

Here's the mystery; Ocasionally, when I open Windows Explorer, I'll find
Drive Y:, Z: or both, each mapped to the shared drive.
I've disconnected them only to find them again weeks or months later.
I'm 98 percent certain that this is not being done by any other user within
our office.
We use a hardware firewall and run Symantec Antivirus.
No other suspicious activity has been noticed.

What can cause this to happen?
Thanks for any thoughts.