I have them syncing with DropBox intentionally.
It asks you if you want the uploads to take place when you first setup the app.
I may not be correct about 'any' android phone though. I've only used stock and several custom ROM on Galaxy Nexus ranging from version 4.0.4 to 4.2.2. After you add a Google/gapps account you'll see this in sync setting- http://s24.postimg.org/fxbv98s05/Screenshot_2013_04_28_23_42... . I've found 'Google Photos' always checked by default. First time this feature was introduced, I didn't notice and my G+ filled up with random images from my mobile gallery. Since then I consciously turn this off everytime I flash a ROM.
Edit: so you don't necessarily need Google+ app installed for this to happen.