So-called "orphaned" photos
https://support.google.com/photos/forum/AAAAzDQC9KE4DBobWatvSE (2015)
https://support.google.com/photos/thread/9842165/sorting-through-1000s-of-photos-and-putting-in-albums
https://support.google.com/photos/thread/82259271/non-assigned-photos
https://support.google.com/photos/thread/86239066/hope-can-you-tell-if-a-photo-is-already-in-an-album-albums
and many others
https://support.google.com/s/community/forum/24053/thread/109903108