I had a similar realization when I found a private photo, so private that I had deleted it from everywhere in some Google's "backup" sites. I am very careful disabling every feature that could mean that personal images or text from my phone will end up "in the cloud" and still it got there through an accident... and a lot of Google's disregard for my wills. It's almost impossible to stop them.
The photo being private is most likely not what you think :)
I deleted it from the phone and my hard disk. I use to take photos with the phone, move through usb to the disk and never publish any photo anywhere.
Where I found it I don't remember because it was some Google site that I hadn't signed up for. I know, they're all connected, but I didn't even know it existed, much less that personal photos could go there.
My frustration comes from the fact that I had already visited several timesevery configuration and options page of the products that I use, choosing not to share anything anywhere with anyone and, god forbid, never backup phone and still it got there.
I have an idea why it could happen: a combination of "sync" option in the phone being activated by an accidental click with a lot of "creativity" in Google's part.
I'm done with this shit. When the time comes to buy a new phone, I will take some drastic measures. Maybe a dumbphone with tethering paired with a tablet, I'm not sure, but not this again.
The photo being private is most likely not what you think :)