If I slave my opponent and he has Cunning Wish (or any wish for that matter), can I look through his Sideboard? And can I "fail to find any instants" even if there is?
On a related note, can i Slave my opponent, cast Demonic tutor, and fail to find ANY cards?
If you search a non-public zone (ala Wishes and Tutors) for a card matching a specific criteria (Mystical Tutor, all wishes except Death Wish) then you can "fail to find".
If you search a non-public zone for "a" or "any" or "some number of" card(s) (Death Wish, Demonic/Vampiric tutors, Intuition) then you have to find however many the effect says to find.
Even if you've slavered someone, his/her instances of non-public zones still are considered non-public.