If Watcher for Tomorrow leaves the battlefield before its triggered ability from hideaway resolves, its leaves-the-battlefield ability resolves and does nothing. Then its enters-the-battlefield ability resolves and you exile a card with no way to return it to your hand.

You don't reveal the exiled card when you put it into its owner's hand.

Any player who has controlled a permanent with a hideaway ability since a card was exiled with it may look at that card.

Hideaway now causes you to put the rest of the cards on the bottom of your library in a random order instead of any order.

"Hideaway N" means "When this permanent enters the battlefield, look at the top N cards of your library. Exile one of them face down and put the rest on the bottom of your library in a random order. The exiled card gains 'The player who controls the permanent that exiled this card may look at this card in the exile zone.'"

Previously, permanents with hideaway entered the battlefield tapped. This ability has been removed from the definition of hideaway. Older cards have received errata to have an additional paragraph that reads "[This permanent] enters the battlefield tapped," and they now have hideaway 4.