Gmail launched a new feature this morning designed to detect suspicious activity in your account and notify you when a suspicious login has occurred in your account.
Now, if it looks like something unusual is going on with your account, we'll 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.
If you see the warning and click to Show details and preferences, you'll see something like the image below:
...which prompts you to change your password immediately if something seems amiss. If it's a legitimate access — say you're travelling — you can simply click click Dismiss and everything will go back to normal.
Detecting suspicious account activity [Official Gmail Blog]