The G-Suite tools support collaboration with users who are not signed in in some cases, like when the file is shared to the public. In the event that a user accesses a file and they are not signed in to a google account, they will be tracked as an Anonymous User inside the G-Suite admin's audit log. We track these events, but can't provide much more detail about these users.
To prevent an Anonymous User from accessing files, you should ensure that no files are shared to the public. See all publicly shared files inside the Altitude Networks app here.
You can read more about the Anonymous User on the google help page here.