A gentleman, in his opinion--and according to d'Artagnan's portrait of him, the stranger must be a gentleman--would be incapable of the baseness of stealing a letter.
Porthos saw nothing in all this but a love meeting, given by a lady to a cavalier, or by a cavalier to a lady, which had been disturbed by the presence of d'Artagnan and his yellow horse.
Aramis said that as these sorts of affairs were mysterious, it was better not to fathom them.
They understood, then, from the few words which escaped from d'Artagnan, what affair was in hand, and as they thought that overtaking his man, or losing sight of him, d'Artagnan would return to his rooms, they kept on their way.
When they entered d'Artagnan's chamber, it was empty; the landlord, dreading the consequences of the encounter which was doubtless about to take place between the young man and the stranger, had, consistent with the character he had given himself, judged it prudent to decamp..