I don't like surveillance, but at the same time, I wonder: How much of this lack of record can be attributed to parallel construction, where law enforcement use

I don't like surveillance, but at the same time, I wonder: How much of this lack of record can be attributed to parallel construction, where law enforcement uses the surveillance results to "luckily" know where to look, and "by chance" figure out how to foil the terrorists? I can imagine that the intelligence agencies may not be thrilled about broadcasting their capabilities. My nightmare scenario is that surveillance actually is helping, but the extent is a closely guarded secret, which means that we'll never get rid of it, and we'll never know why, or have a chance to, as a society, have a conversation about the tradeoff between surveillance and counterterrorism.