Define 'hidden'. If the samurai is engaging in a personal challenge, and going to fight with a weapon, he'll take it out. If he's going to walk into a corporate facility he's trying to infiltrate, of course he'll keep it hidden. If he's engaging in a personal challenge and he has no intention to fight with a weapon - the pistol in question, I believe - then he won't bother taking it out, because he's not going to fight with it.
However, I should note that samurai were more than willing to pull a knife that was worn behind their back (and thus 'hidden') in the middle of a fight, because the only true dishonor is to fail to obey one's honorable lord.