I agree with gfarb that the claimant does not necessarily believe in a belief; taking the parable literally, it is far more likely that the claimant suffers from a delusional disorder and actually genuinely believes that there is a dragon in his garage. As to why he anticipates that no one else will be able to detect the dragon, that would most likely be explained by his past experiences, where other people denied his experiences and failed to find evidence for them.
I agree with gfarb that the claimant does not necessarily believe in a belief; taking the parable literally, it is far more likely that the claimant suffers from a delusional disorder and actually genuinely believes that there is a dragon in his garage. As to why he anticipates that no one else will be able to detect the dragon, that would most likely be explained by his past experiences, where other people denied his experiences and failed to find evidence for them.