Can Google use location information based on Photos landmarks with Location set to off?
It doesnt look like they can do it if your GPS is shut off in your settings. 

"If you turn off location completely for your device, it means that apps (both Google and non-Google apps) can’t use your device location and many useful features will be turned off."

As to them checking it in app. Seems like that is something they are trying to accomplish. What i have found to disable it is if you go to Settings-&gt;Google Location Settings and uncheck Access Location then they can't access your location even if your GPS is on from that app. That solved that problem for me.

I have found some information for you in the below links.

Google Landmark Recognition.pdf

Turn location on or off for your device