When Google Photos was released last year during Google I/O 2015, people were understandably a little wary of Google's free service - for those looking to use the High-Quality setting, at least. After all, to work as advertized, Google Photos needs to collect quite a bit of data. This mostly involves the photos you upload being passed through a neural network, think artificial intelligence, which figures out what is a cat and what is a tree, for instance. Then there's location data, which only really applies to either cameras that put GPS coordinates in their EXIF data, as well as smartphones that have it enabled. Recently however, some users have been noticing behavior that looks as though Google Photos is looking into someone's location history in order to retroactively tag a photo with location data.
In one example, a user discovered that photos uploaded from their DSLR was tagged with an "Estimated Location" which appeared to be location taken from his phone during the same sort of time that he took the uploaded photo. The plot thickens however, when we learn that this particular user never had the location option turned on inside of the camera app, and even with this option turned off, their photos were still given "Estimated Locations". This behavior doesn't seem to be something too new however, as of October 2015 it appears that support pages hosted by Google detail this behavior, and Android Police were given the following statement: "For photos that do not have location information from your phone/camera, we now might estimate a location for that photo using information such as your Google Location History."
For a lot of people, this might not be a big deal, but if you told a camera app you didn't want the location recording, this behavior on Google Photos' part sort of goes against that. We suppose that users who really want their location information to stay private on their photos will now need to turn off all location tracking, which seriously hinders Google Now and other apps from working at their best. This does seem a little strange, but Google Photos is designed to make life as easy as possible, and if you were looking for all photos from a specific place, and used a camera without GPS, this would certainly make them easier to find than searching for the exact time or date.