It is extremely useful for Fred to know a lot of quick exits from sticky situations. His extensive knowledge of secret passages in his own castle means he can recognise the subtle signs of their existence in others. They don’t seem to be common knowledge though. I think Fred and George found them through their natural curiosity, although maybe they learnt about the existence of one or two from Ludo, their cousin, who they admit to playing with when they were very young. Ludo seems to use his knowledge of them to live a double life.
Lupin is well aware of the secret passages in Buckmore. Lupin advises Fred of a couple he hadn’t already discovered before he goes back to investigate the Lost City of Arbor, because it is modelled on the same design as Buckmore. Lupin suggests that the tendency of castles to use the same pattern extends to their secret passages, although I’m sure that some castles have passages that are unique to them, devised by earlier occupants or responding to a prince with the special gift of ‘needing’ the castle to give him an exit. Fred’s escape via the statue in the main entrance hall (in Pirates) seems to be a new or previously unrevealed passage in response to his need. Since he turns up back near his cubbyhole I suspect it is a new one. I wonder if these are sometimes transitory, closing when the need is past? Fred and George seem quite pleased to find the entrance to the tunnel to Seventh Happiness still there when they return after some months’ absence. By the time we get to book 5 it is clear that while that quick exit is still usable, there is a new permanent entrance to the Marsh tunnel, used by regular travellers with coaches, even if the stage travels via Wash.
Mariusz is another who makes substantial use of quick exits provided by secret passages, especially in Sowerby. He has help finding all of his though, or else he discovers them by chance. It appears that he doesn’t need this skill in Hattan. Or maybe the different construction of their towers means that secret passages are a thing of the past. Shame.