There's an MS document, KB811660 that talks about suppressing the warning for particular file types and locations. Its a bit obscure but the key point is to add this to the registry to skip warnings for Outlook PST files.
HKLM\Software\Policies\Microsoft\Windows\NetCache\ExclusionErrorSuppressionList \\\\*\\*\\*\\*.PST = 0x0000000 (REG_DWORD)
Unfortunately that didn't clear the error, even after the obligatory reboot. The trick to get it to take effect was to reset the Offline Files cache too. Go into My Documents > Tools > Folder Options > Offline Files and use Ctrl-Shift with Delete All Files. For more info take a look at KB230738.