They will attack armies in the open, if it is damaged or weak defenders (swords and MDI). They will rarely stumble into an army, if you have the nopatrol enabled. I have only seen that twice in countless armies in the field (in some games I have had more than 100 armies). I have not seen them ever attack a knight armies that was not low yellow or red, in the field. In towns, they have attacked even MI armies. Towns are different set of circumstances.