I do believe this came up once before, though it was most likely in an episode discussion thread.
We've never seen yorouchi's zanpaktouh, and there's never been any indication that she hides or or lost it or whatever. I've always figured she never really had one. Or rather, she had some other manifestation of her soul (or whatever a zanpaktouh is) instead of a sword. The shikai and bankai of the various shinigami usually seem to represent thier inner selves, and the actual sword is just a container or sorts (remember, even the lower shinigami have swords. they just dont have shikai's).....for yorouchi, i suppose that inner power is well.....in her. Maybe the whole turning into a cat thing is her shikai? i mean, no other shinigami seems to be able to do the shit she does. That power had to come from somewhere.
Another possible (and more probable) theory is that she never had a sword as she's not really a shinigami.....atleast not in the normal sense. If i remember my bleach history correctly, she was the captain of the black-ops squad or something. Not the same as the gotei 13, like all the other captains. I dont think she or any of her squad ever went out hollow hunting like the rest of the shinigami. They purpose was something else (i dont know if it was ever stated what they actually did).