Unmaksed
From the MSM FAQ:
Players can discard cards to power-up their characters in response to Unmasked if it's played during an attack.
If Unmasked has already resolved, then you cannot power up. You would have to power up in the chain before (the non-ongoing part of) Unmasked resolved.
Searching
Comp rules 5.13.2:
Some one-shot modifiers instruct a player to search a zone. A player may fail to find, either by choice or absence, any card or cards that he or she is instructed to find by this modifier. The player does not have to state whether the failure to find was by choice or by absence.
Example: Faces of Doom reads, "Search your deck for a card named Dr. Doom. Reveal that card and put it into your hand. Shuffle your deck." A player plays Faces of Doom. That player has a Dr. Doom, Diabolical Genius in her deck. She may choose not to find this card.
So the opponent would not go through the deck if the card was "not found".