Well, of course it depends on the type of game and context, it just sounds a bit like an uninteresting tool, to just have a just invincible NPCs. I think special weaknesses make every NPC more interesting. Give the NPC a weakness (eg. Superman hs cryptonite or Lord Voldemord has Horcruxes).
Im curious too. Why shouldnt a boss be invisible... sorry, invincible ? Especially when you have no clue of the context except that it went away and the players survived ?