How would they know? Did they inspect all the military sites the Mullahs said couldn't be inspected?
Again, the Iranians complied with the treaty, they complied with inspections, and teh billions we've spent on intelligence didn't fine one shred of proof they violated the treaty.
The Europeans worked very hard to get that treaty to start with, and Trump tore it up on them.
Under the treaty, there were not sites that were "off limits" All nuclear sites were subject to immediate unannounced inspection. While non-nuclear sites, were subject to a process which could give them up to 3 weeks to gain entry to.
It would be like Russia wanting immediate access to the Pentagon, or to NORAD in Cheyenne Mountain, without time to remove or lock up classified information.
BTW: Given 3 weeks to remove nuclear material doesn't remove the traces of nuclear material, which as Chernobyl proves, remain for decades or centuries, and can't be washed away.