Legal Question in Criminal Law in California

I hope you have a lot more details than that. Analyzing whether a search and seizure is lawful is heavily dependent on the facts. You don't provide any information as to whether a warrant was obtained, when the search occurred, whether your car was stopped by a police officer for a traffic violation or other issue, whether the search was done in the parking lot of the store, whether the search was incident to an arrest, if anything was found, whether you consented to the stop, who the witness was, etc.

You should speak to a criminal defense attorney, and have them analyze the facts to determine whether it was lawful.

Search by who? The police must either have a warrant or some exception to the warrant requirement. There are somewhat relaxed rules when it comes to vehicle searches, but they still must have probable cause to search.

The store personnel typically don't search cars, but let's say they did... unless they were at the time acting as an agent of the police, they aren't under the same Fourth Amendment rules as law enforcement.

Search questions are always fact-dependent, so you need to sit down face to face and discuss this with your attorney.