We can understand his taking an evening stroll, but the ground was damp and the night inclement.
Is it natural that he should stand for five or ten minutes, as Dr.Mortimer, with more practical sense than I should have given him credit for, deduced from the cigar ash ?" "But he went out every evening." "I think it unlikely that he waited at the moor-gate every evening.

On the contrary, the evidence is that he avoided the moor.

That night he waited there.

It was the night before he made his departure for London.

The thing takes shape, Watson.