> To help minimize damage from unauthorized access to your account,
> Google alerts you when someone signs in to your account from an
> unusual location. Their goal is to help you secure your account
> quickly if an unauthorized person has access to it. This especially
> also happens if you sign in to your account from an unusual location
> and even make a wrong password login attempt. They alert you about
> sign-ins that are from locations and devices that they previously have
> not seen signing in to your account and ask you if it was you who
> signed in from those locations or devices.  To help you determine if
> it was you, they show you the information they have about the
> suspicious sign-in event(s) including the IP address, its location,
> the domain associated with the IP address and the time at which it
> accessed your account.

If the location was familiar, then do not worry. If not, I advise that you change your password, update your account recovery options and run a virus scan on your computer.

More info is available here: [Suspicious sign-in alerts][1]


  [1]: http://support.google.com/accounts/bin/answer.py?hl=en&answer=1144110