Gmail freaks out users by saying a 'new device signed in to' their account

Don't freak out. 

On Monday afternoon, Gmail mistakenly sent multiple users a security alert, notifying them that an unknown device had signed into their account. 

The number of affected users is currently unknown. However, according to a G Suite admin who chose to remain anonymous, the issue was widespread enough for Google to issue a notification about it in its Admin Panel. As of this writing, and according to the admin, Google was investigating the matter and said it had identified a potential cause, though it did not specify further.