In this day and age where we create site collections instead of sub sites it’s quite easy to see who has taken an interest in your stuff.
Header over to https://tenant.sharepoint.com/sites/yoursite/_catalogs/users/simple.aspx and view the contents of the hidden User Information List. This list contains an entry for anyone who has ever visited your site.
And a shout out to Tobias Zimmergren and his trusty old post from 2008 covering this http://zimmergren.net/technical/sharepoints-hidden-user-list-user-information-list.
