This is the third part of a quest chain, and you get it after completing The Defias Brotherhood II. Speak to Gryan Stoutmantle in Sentinel Hill, and he will say:

I wonder what Wiley meant when he mentioned the Stonemasons.  Perhaps that was a slip of tongue.  Could the Defias gang be related to the Stonemasons?  Only one man would know for sure:  Mathias Shaw, head of SI:7. Show him Wiley's Note and see if he has anything to add to this growing mystery. If you have trouble finding Shaw, check the Barracks in Old Town.

Travel to Stormwind
Speak to Mathias Shaw

Off you go now! Back to Stormwind, and here you will find Shaw in the Old Town District, inside the Barracks on the second floor. Speak to him and he says:

What business do you have with me? I am a very very busy man?

This matter might be more complex than Stoutmantle realizes.

Now we?re getting there, some interesting info is about to turn up! So if you?re curious, check out The Defias Brotherhood IV.