When using certain authentication methods, it might be that a system hostname or computername shows up in the web filter logs instead of a username.
There are always system processes running in the background on any computing device. Some of these processes might try to access the internet via the web filter. If the web filter asks for authentication, the resulting "username" given out, is the system hostname or computername rather than the currently logged in user, if any.
This depends on what process is causing this and what the process is accessing. Please contact support for further advice on how to minimize this issue.