where would i find a ‘custpage’ field in customisation?

Those are typically added dynamically via code

Ah. we’ve gone live with SuiteApprovals, and the ‘next approver’ is a custpage field. We can’t reference that field via saved searches, I’m guessing that’s why, it’s a dynamic value that isn’t part of the form?

It's probably added to the page via code/workflow dynamically, not part of the actual definition of the record itself