It has been more than three years since President Obama revealed the existence of the secret Iranian nuclear facility at Fordo—a uranium-enrichment plant buried deep inside a mountain and surrounded by missile silos and anti-aircraft batteries. Is the world due for another surprise soon?

If anyone has standing to speculate, it is Olli Heinonen, who says he first “got a whiff” of Fordo six years before Mr. Obama acknowledged it. In the fall of 2003, Mr. Heinonen was in his office at the International Atomic Energy Agency in Vienna when a man appeared unannounced. The “walk-in”—whom Mr. Heinonen hasn’t previously discussed, and whose nationality he won’t disclose—claimed that Iran was replicating its existing uranium-enrichment facility in an underground site near the holy city of Qum. And so it was, as the IAEA and Western spy agencies later confirmed.

But that isn’t all the walk-in shared in 2003. Also under construction in Iran, he said, was a duplicate of the Arak heavy-water facility designed to produce plutonium. In other words, the walk-in said that Iran had at least two secret sites, and he was correct on the first. What about the second—is there a plutonium facility that remains secret today?