Can a token be returned to its owner's hand and what happens when it leaves the battlefield that way?

Tokens are permanents while they exist on the battlefield, and they can absolutely be targeted by effects that return permanents to their owner's hand — such as Unsummon or Boomerang. There is nothing in the rules that prevents a token from being bounced.

However, the moment the token leaves the battlefield (in this case, moving to its owner's hand), a state-based action immediately causes it to cease to exist. This is defined in CR 704.5d: a token that has left the battlefield is put into the appropriate zone and then ceases to exist. The token never truly 'exists' in the hand — it arrives and vanishes simultaneously before any player can interact with it there.

This has an important consequence: the token cannot be played or cast from the hand, it cannot be discarded by a later effect in the same turn, and it does not count as a card in hand for effects that care about the number of cards a player holds. It simply disappears.

Concrete example: Your opponent controls a 1/1 Goblin token and you cast Unsummon targeting it. The token moves to its owner's hand as Unsummon resolves, and then state-based actions are checked — the token ceases to exist. Your opponent's hand size is unchanged, and the token is gone for good.

The same applies to any zone a token moves to outside the battlefield: graveyard, exile, library, command zone — in each case, CR 704.5d and CR 111.7 ensure the token ceases to exist as soon as it arrives there.