One of ways the US intelligence services determined that Iran had ceased its nuclear weapons program was by intercepting phone messages. Again, from the Telegraph:
British analysts believed that Iranian nuclear staff, knowing their phones were tapped, deliberately gave misinformation …They say things on the phone because they know we are up on the phones.
Intelligence officers have said for centuries that gathering information is one thing. Understanding what you are looking at is much harder. Determining what your adversary's intentions are, based upon that information, is the most difficult of all.
One of the reasons the US and USSR allowed each others' spies to roam around inside their borders was because it was in each country's national interests to have confidence that the other was not about to launch a nuclear strike.
The Iranians have not backed away from enriching uranium for the stated purpose of building nuclear power plants. But, material produced during that process is the precursor of weaponized uranium - that is the stuff of bombs.
Like everyone else, I hope the Iranians have indeed stopped trying to build a nuclear weapon. But as the former Israeli deputy minister of defense told the Telegraph recently:
"No one can rule out with high confidence that somewhere in Iran, 70 times the size of Israel, there is one lab working on the weapons programme."
That's the black hole of the intelligence metric (what we know, what we don't know and what we think it means) as reported by Reps. Hoekstra and Harmon.
There is always the nagging doubt - as a former US Secretary of Defense once famously stated - that "we don't know what we don't know."
And when it comes to Iran, we should be very aware that we don't know a great deal.