Legal Question in Criminal Law in California

That is going to depend on more factual detail, which you should share with a criminal defense attorney in private. Police executing an arrest warrant in a private residence have a right to do a pat down of people they encounter in that residence to make sure that they don't get stabbed in the back while arresting someone. They can also seize contraband in plain view while affecting the arrest, but only in plain view where the officer is making the arrest (the Chimel doctrine.) Whether or not this happened in your case will have to be evaluated by a criminal defense attorney.