I've recently turned on the setting to be notified when an unrecognised device logs into my Facebook account.
However, I think that someone has used my account on another device BEFORE I turned this on. If they attempt to login again, will I be notified or will their device be classed as a recognised device?
If they've logged in successfully before from that device, there's a good chance it will be classified as a recognized device.
However, you can see which devices your Facebook account is currently logged into, and you can log yourself out of any of those devices. From the dropdown menu at the top of the Facebook site, go to:
If you see any unrecognized devices there, or devices that you know aren't yours, log yourself out by clicking the three dots to the right, then Log Out.
Logging out of all current sessions, combined with changing your Facebook password, will require anyone logging into your account from another device to know your new password in order to log in as you again.
