Let me split your question into three different questions.
Is there a way they can hide their IP address?
Yes it's possible to hide your real address from other people or companies. You just need to use a VPN and switch to your desired location. It's that simple. If you want to read more about VPNs I got an interesting article here
Does Facebook only show most used IP addresses?
No, Facebook lists all logins chronologically. You just need to login and navigate into the settings. After that go to "Your Facebook Information" > "Access your information" > "Security and Login Information" > "Logins and Logouts" now you can navigate down and on the last entry you will see a list of all logins. Maybe this helps you.
Note: I think you can't find the full log because you are only at Security and Login but there is only a list of currently logged in devices!
How can we find out for sure?
If you find different IP addresses and login timestamps in this log you will know that you didn't log yourself in. Maybe you could look up these IP addresses and see that it's not even in your country, but don't trust it. As mentioned before it's possible to use a VPN to make it look like the IP is from your location.