I was playing Avernum 3 again recently, and I rediscovered how tedious it is to be constantly searching for secret passages (even though I have remember "roughly" where they all are from many previous playthroughs). The same basic technique applies, i.e. look at the automap to spot where a secret area is likely to be, then figure out how to get in there. But the Avernum 5 setup is harder, because the switch need not necessarily be adjacent to the secret area -- this adds more of a challenge. And it may also be hidden by the perspective. And Avernum 5 has much darkness, so the dark spots tend to blend in unless you use lamps and walk very close to the walls.
I didn't find hitting "u" all the time to be a good way to spot switches... too many barrels, chests, bodies, boats, etc, makes it way too tedious.