I realise this question is somewhat of an intersection of OS X and Google but I'll start here. My partner discovered that most of her pictures (up to 2013) that are on her Mac were mysteriously available on Google Photos; we're puzzled as to how all these pictures ended up on Google Photos when we did not explicitly upload them.
What mechanisms are available that would automatically sync up photos from OS X? My prime suspect is Google Drive for OS X. We don't appear to have that installed any more and that might explain why the pics only go to 2013.
What other items can we look at? They're not pictures from her phone (iPhone 4, iOS 7) nor have we installed anything on there that might sync pics from her phone.
Best Answer
Google + for iOS, Google Photos for iOS and Google Photo Autobackup do that. The age of the pictures is not really relevant, because the last app would upload any image on your computer (on the pictures folders mostly). Also, Picasa used to be able to sync your pics with G+, although I'm not sure it was automatic.