"This account is open in 1 other location at..." Gmail user interface provides this message when your account is used in multiple locations simultaneously. One of the most important and interesting security feature that any web mail service would wish to provide. I suppose Gmail is the only web mail service that has this type of security feature (let us know if any other does). Users become aware of any misuse of their Gmail accounts.Gmail is countering the number of active sessions per account in providing this information; so "open in multiple locations" means that there are multiple active session for this particular account. Gmail also provides more details about each session making it easy to track the misuser. Access Type, IP address and Date/Time are displayed as details.
Effectiveness
All these details are visible to all web sessions including misuser's session; so the effectiveness of this feature is in doubt. For example, a misuser can sign out as soon as this information is displayed in Gmail account. However a solution for this will not be simple and easy. In the meantime, these information is not displayed in a very prominent place (shown at the very bottom of the page), only the misuser will use these information rather than actual (innocent) account owner.Improvements
We are not highlighting the weaknesses by neglecting effort made in providing this feature. However, as an improvement; active sessions count can be shown at the top bar; so that all users will notice the multiple session usage. However this should be displayed only when multiple sessions are active.