"Your device has been modified no updates are available" huh?

Was checking for software updates today and got this message "Your device has been modified no updates are available" , checked device status and it says "Custom" not "Modified"?

I bought the phone brand new but it was unlocked unofficially I think because it has a boot logo of a different carrier.

Whats going on here? Surely unlocking the phone doesn't "modify" it?