Here is a useful page link with information on how to perform a Facebook remote logout. This allows you to log out of any Facebook session that you may have left active on another computer or device.
Facebook Remote Logout
This feature will also let you see if you’re still logged in on other devices and immediately log out on those devices from one central control in your account. This feature is rolling out gradually. Once it’s live for you, you can try it under the “Account Security” section of your Account Settings page on your Facebook account