I played with Ashiok 1 and stole opponent’s Primal Adversary with Ashiok’s 1st loyalty ability. When I exiled Mind Flayer from my hand, Primal Adversary was returned to opponent’s side.
Although the description of Mind Flayer says that the non-owned creatures are to be returned when Mind Flayer dies or is exiled from the battlefield, they are also returned when the Mind Flayer is exiled from hand either voluntarily by a player or by an effect.
I assume that it might falsely trigger also if Mind Flayer is destroyed in player’s hand, for example by Infinite Obliteration.