Google account verification request
Google says it's not a security problem and that you don't have to worry.
After investigation they issued a statement in the Google product forums:
What happened?
During routine maintenance [from 1pm to midnight PST yesterday], a number of users were signed-out from their Google accounts. This may have resulted in you being signed out of your account or seeing a notification about “A change in your Google account” or “Account Action Required.”
We hear your concerns that this appeared to potentially be phishing or another type of security issue. We can assure you that the security of your account was never in danger as a result of this issue. You can always learn more about the Security tools Google provides at www.google.com/safetycenter/
What should I do now?
First, try signing back in with your usual username and password at accounts.google.com. If you can’t remember your password or can’t sign in for another reason, recover your account here: g.co/recover.
Note that the statement only refers to this particular incident from around February 24th, 2017 which has affected many users and is likely harmless. But more generally, if Google is asking for a security confirmation or warning you about changed account details, you should take that seriously and review your security settings and recent logins - as you should do regularly anyways.
Google released a statement earlier today, confirming that it was not a security breach. I found an article here
Here's their statement:
We've gotten reports about some users being signed out of their accounts unexpectedly. We're investigating, but not to worry: there is no indication that this is connected to any phishing or account security threats.
Please try to sign-in again at accounts.google.com and if you cannot remember your password, please use this link (g.co/recover) to recover your password. If you use 2-Step Verification, there may have been a delay in receiving your SMS code. Please try again or use backup codes.
If you're worried about the legitimacy of a login notification, i.e. if you suspect that the notification might actually be from a malware, then don't sign in on the password prompt on the notification bar. Instead, go to one of the Google Apps or System settings and trigger the login dialog from there.