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.
A "new device sign in" notification is a freaky message to get, because it means that someone else has already successfully signed in to your account. Gmail informed users to change their passwords "right away." Read more...