You may remember that a while back GMail launched remote sign out and information about recent account activity to help you understand and manage your account usage. This information is still at the bottom of your inbox. Now, if it looks like something unusual is going on with your account, GMail will 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.
To determine when to display this message, our automated system matches the relevant IP address, logged per the Gmail privacy policy, to a broad geographical location. While we don't have the capability to determine the specific location from which an account is accessed, a login appearing to come from one country and occurring a few hours after a login from another country may trigger an alert.
Official Gmail Blog: Detecting suspicious account activity
