"Probably to counter balance the good luck of managing to evade me for so long," Porthos remarks, because he still intends to make d'Artagnan pay for this. He hasn't tried to find the young man as hard as he could have, but that's only because Porthos would rather make him pay once he has shifted back, once he knows for certain that, yes, this was only temporary. He doesn't want to make d'Artagnan pay for the fears he barely acknowledges, because that way lies madness.