So yeah, what spyfy said.
I am not a lawyer, so this is not legal advice and worth about as much as you paid for it (maybe less). The omission seems unlikely to cause you major problems, especially because you had not used the passport at the time you applied for your PR card. So I don't think there's a reason to go talk to a lawyer right now, but it's your call.
Misrepresentation is an issue particularly
if it could have changed IRCC's decision ("material misrepresentation"). The passport not having been used means there's no absences from Canada that you hid from them, or trips that could raise security concerns, or anything of that kind. So I think that is not likely to be a problem.
IRCC may give your application closer scrutiny when they realize you omitted something from a previous application (if they feel it suggests you may not be completely reliable), but that doesn't mean your application would be denied or something like that. It would mean they may ask you for additional documentation to prove the claims in your application are true and complete.
IRCC may not do that. They do know that mistakes happen, so they may just consider it a minor mistake and nothing will come from it. We have no way to predict what they'll decide to do.
(If you want to know what kind of information IRCC tends to ask for, you can read some posts about RQ and PPQ. There's a thread about those
here. Some people gather that information once they submit their application, just so they have it all on hand, in case a request arrives. That may be work that turns out to be useless if IRCC never asks for the information, so whether or not you want to do this is your choice. If you do gather the documentation make sure it covers your entire eligibility period, so know that some documents can't be requested until you've signed your application.)
