It's an interesting question- contraband will never go down unless you cut off access to it altogether, which of course means cutting off access to certain rooms and amenities. The reality is that contraband will stay high but regular shakedowns and searches will keep the numbers steady and somewhat manageable.
Shakedowns also aren't totally successful. They have a success rate somewhere above 50% (can't remember the actual number), to reflect human error. There's every chance that a guard performing a shakedown simply won't notice a tunnel even a couple of searches in a row. That's just how it rolls!
Nat D11