Comment by arturhoo

I can't find any evidence that it has been. I use a long and strong password and have 2-step-verification enabled in my account. I haven't received any new account activity emails and the I don't see anything suspicious on myaccount.google.com

As far as I remember, I've never used Panoramio (as a comment above suggests to be the cause). I'd agree that this seems limited to some specific circumstances.