Gmail is introducing a new feature to notify you when we detect suspicious login activity on your account. If it looks like something unusual is going on with your account, well also alert you by posting a warning message saying, “Warning: We believe your account was last accessed from along with the geographic region that we can best associate with the access point. If you think your account has been compromised, you can change your password from the same window. Keeps in mind that these notifications are meant to alert you of suspicious activity but are not a replacement for account security best practices.”]
Source: https://security.googleblog.com/2010/03/detecting-suspicious-account-activity.html