Personally a bit skeptical of this "...we don't know where the HEU has gone..." line.
For a long time now US IMINT platforms (both UAVs and things that fly or orbit at higher altitudes) have had the ability to `stare' at large areas of the earth for long periods and record what they see in exquisite detail.
The HEU can be carried in canisters which will fit in the back of any military vehicle. There will literally be hundreds of such arriving and departing from the sites daily loading and unloading out of sight.
Once a vehicle has left a site it can stop in any tunnel or any loading bay or garage and the contents moved to any other vehicle and moved elsewhere. A tactics which can be performed multiple times.
The idea that the presence of the HEU could be confirmed, let alone tracked, by satellite or drone surveillance is something reserved for Mission Impossible movies.
The only one’s I’d suspect have any idea of it’s location, based on HUMINT, not ELINT which I suspect the Iranians will be paranoid about in such circumstances, will the Israelis with people on the ground who have infiltrated the programme over the years.