Did you by any chance remove browser data while this happened? (even if it was from another device, using the same google account)?
No, I very rarely do that.
I have two active login sessions (always logged in), on two devices and one browser type (two different Google accounts), but everything is completely under my control. It's been working fine until today; this incident happened on both of my devices.
Or do you have some software like CCleaner that does the cleaning for you on a regular basis?
Nope.
It is better you change your password and email to be able to know that no one is having access to your email and login password.
I don't use Tor.
Thanks for the reminder, but I don't plan on doing that right now.
Did you log out on a *different* device?
No, I rarely use the logout button.
Edit:
But in this case I will tell you to check your ip log carefully.
I've checked, there are no strange locations.