I'll stipulate that when faced with an ambiguous stimulus, the human mind often fills in the unknown with what it expects (or hopes) to see.
Consequently, person A, expecting to see a flying saucer, is more likely to see a flying saucer when faced with an unknown or ambiguous visual stimulus.
Person B, not expecting a flying saucer, is much less likely to see a saucer when faced with an unknown or ambiguous visual stimulus.
I guess the question is, how do we best study the experiences of people who didn't expect to see something?
I'm just glad I haven't met one.
;-)