Indeed it is not, however it's quite difficult to scrub anything sufficiently to hide them from IAEA environmental sampling (most of which is chemical based, primarily micro scale spectroscopy). Enriching uranium is a nasty, dirty process at all steps that leaves a lot of sticky, messy, and hard to get rid of traces, and IAEA has access to some of the most sophisticated analytical laboratories on the planet and a lot of practice.

If the inspectors are given sufficient access to do their inspections.