My guess is that every truck for the last month or more has been tracked via satellite (and knowing Mossad via on the ground GPS locators as well), and they already have a solid idea of where it may be stored as a backup.
The real problem is that we are talking about is at most a few hundred kilograms of enriched material, so tracking it is more problematic than just burying it under the rubble.
The only way to get Iran to try to transport out of Fordow was to make it so that getting hit was imminent. Now what is interesting is was the timing of the Guam flight was done on purpose, knowing the Iranian evacuation plan for an impending US strike? Is it better to bury the current enriched uranium deep under Fordow and hope no one can recover it, collapse it near the surface (and therefore need boots on the ground to recover it) or get them to transport it and capture it either in route or at another location.
I think Israel and the US were hoping the very visible head-fake of the B2 flight heading to Guam got Iran to stage whatever they had left in the facilities in the transport tunnels waiting to depart in the morning, which instead got struck at 2 am local time. That puts it reachable with some work, but easily policed by air to make sure it "stays put" by not allowing any re-excavation until the time that either there is a regime change, a deal in place that requires that material to be recovered and turned over under US/Israeli supervision, or Israeli boots on the ground.