[Endoperez]

Some special summoned commanders cause disease and destruction, and atleast one is stealthy. I do believe that he would be found on your home province with its prov. def. of 25 though.

Also, the mscellanous item Bane Venom Charm has the same effect, so it might just be that item on someone in there causes it. Check your commanders and for Stealth +X commanders.