I think tamed boars can aggro on enemies and follow them if not fenced in. Otherwise, they seem to have an anchor point once fully tamed, learned that the hard way when trying to move them to a pen too far away.. So they shouldn't run away from that point on their own under normal circumstances.

As for the slipping out, I have witnessed this sorry event. Now and again, piggys are born squeezed through the fence. Couple days later, *poof* you find a tame boar outside of your enclosure.