We find out, becasue JA, through Eleanor, tells us that Henry meets Catherine first whilst he is scouting lodgings for his father- he doesn't stay in Bath, but is constantly flitting in and out of it- back to his parish. That's why Catherine keeps looking for him and not finding him.

Neither does he stay at Northanger- he regularly goes back to his parish at the weekends, and explicitly for Mondays (I'm guessing that's for the Vestry meetings). He stays at Northanger beacsue he is commanded to do so by his father, as well as because he wants to see Catherine. In normal circumstances,I think it's highly likely he spends much less time at Northanger, despite the fact that his curate is competant.