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.
By clicking on the "Details" link next to the message, you'll see the last account activity window that you're used to, along with the most recent access points.