The same issue here, the last user is most of the time not properly recorded.
It seems that a computer administrator is properly recorded but not a standard user.
I am talking about users on Windows 2000 OS, XP and Server 2003.
This issuea appeared after one of the recent updates.
The last user logged is not refreshed even with triggerscan, rescan, full rescan...
Database schema: 13
Website version: 4021
Sql 4.1.0.29