From reading the comments, I thought I was going to be in for a long, frustrating drop quest.

But it turned out I was lucky too.

When entering Stromgarde, pass underneath a bridge and take a left.  You should see a patrolling Syndicate theif and Syndicate caster with her minion.  I killed both of them, and the Sigil dropped off the theif. 

Solo'd as a 35 hunter. 

p.s., beware of the patrols that enter the keep every once in a while.  Normally there are three on horseback. 

Good luck!