to their owner's hands

So it return in his hand.

But the Soul Stealer card says:

"Take control of attached unit. Move that unit to your corresponding zone."

So if I'm the one who controls the unit doesn't it mean that I'm the current owner of that unit?

If the card is in your deck at the beginning of the game, you own it. By default, you control all cards you own, but control can be changed by card effects. Ownership can't. If you attach Soul Stealer to my unit, you control the unit, but I still own it.

Soul Stealer wouldn't even need to say "to their owner's hands", because when a card goes to an out of play zone (hand, deck, discard pile), it always goes to it's owner's out of play zone. There's simply no way you can end up with an opponent's card in your hand.

Thank you for great explanation.

Didn't know that there is a difference between owner and controller, but now it makes sense.

P17 of the rule book :

Control and
Ownership
Players “own” cards that they included in the deck
they are playing. Players “control” cards that they
own, unless another player steals control of the card
through a game effect. Any time a card leaves play,
it reverts to its owner’s hand, deck, discard pile (as
directed by the effect forcing the card out of play).