Unknown Guest Contributor in SharePoint Online

Hello,
I have discovered that a "Guest Contributor" has edited a file in a customer's SharePoint Online.
We suspect that this "Guest Contributor" has unauthorized access.

Is there any way to find out more about this user e.g. logs etc.?

I can only find a generic ID when I open the user in SharePoint.nt. I have already searched in vain in Entra ID.