It's much more mysterious than an unsolved mathematical problem. Use elements which a hypnotized subject negatively hallucinates. Indeed, there is also post-hypnotic suggestion, and psychological manipulation to hide things in plain sight. Thereby, the person's alertness is broken, or the person's attention is pointed sharply. Then the person is blind to what is to be concealed.
Psychological manipulation infringes the target's freedom even further.
A. Look in the first room you haven't looked in to see if the Holy Grail is there. If it isn't, then these instructions are void. But if the instructions are void, then absence of the Holy Grail has no significance. But then the instructions are not cancelled, and are again operative. If you believe in infinities, then self-apply the work an infinite number of times, changing 'an' to 'the'.