As a counter point, I woke up today to this weird email:
https://bit.ly/2wMpPL3Picture is from 2011 and was originally hosted on Picasa (but I'm pretty certain it was never shared publicly) and I don't have a Google Plus account. If I go to the picture itself on Photos, there's no sharing/privacy controls, so I'd assume it is private.
The wording on the email makes it even worse, as if I had intentionally done something overnight to share them on Maps.