If there are drugs found in a person's car and weren't charged with anything drug related, do police have to bring it up in court?

It really depends on the specifics and the type of case.  They generally bring up issues that are relevant to the matter at hand.  So if it's a criminal case or child welfare, maybe they do.  If it's a breach of contract case, probably not.  You can also address these issues in preliminary evidentiary motions if you are concerned.