Access log is showing user name as "Unknown User" on open link which have visibility is either "Company" or "Open".

I’m looking for how I can identify source of access about below log.

I have multiple access log on open links which is showing “Unknown User” as access source user.

It is easy to understand that if the link's visibiity setting as "Open", but link's which have visibility "Company" also have "Unkown User" access log. Does anyone can help for identify source of access?