Why would they have been able to piece it together? He works through proxies and has kept a relatively low profile. Graendal would know if he was in Shara as she went an captured their leaders, and what is to be gained in the Isle of Madmen?

Everything we know of the place tells us it's a backwater that has never developed because of the rampant madness of unchecked male channelers for the last 3,000 years.

What use are insane male channelers to a General who is likely to see only a weapon that will harm him as much as help him, an uncontrollable weapon is useless. And, it wouldn't take him all this time to grab a few hundred insane men and place them throughout the westlands to sow chaos, he could have accomplished that in a few weeks!

IMO he's playing the "long con" someplace and covering his tracks well. Assuming the role of a backwater powerless king is a perfect hiding place.