Welcome to GB.
I'm entirely sure on this, but I think the Potions of Master Thievery (the purple one) may help temporarily increase the Find Traps/ Hidden Doors skill. If not check around the merchants for weapons that do increase while equipped.
If you stand by the secret wall for long enough it may appear, but that will only happen if you have enough skill points in there.
The Crypt is filled with Hidden Doors so you may want to invest more points in the skill.