Blank (unnamed) user accessing NT 4.0 sh

Server Manager (and netwatch.exe) shows a user without a name simultaneously logged in at several machines accessing network shares all over the domain. All clients are NT Workstation SP4 or higher. PDC and BDC are NT4 Server SP6a. NO DHCP, but WINSis running on the PDC. No user has local access privilege to any workstation ? everyone must authenticate to the domain. The unnamed user has both read and write access to various files (no pattern) in each share. Are there any system accounts that don?t identify themselves? What are some possible explanations?