I don't know the laws of Louisiana, and as has been repeatedly stated on this forum, the laws of every jurisdiction vary. However, in Mississippi, a law enforcement officer MUST make an arrest to seize a firearm, and a record of the seizure MUST be entered on a docket required to be maintained by the agency. Failure to either maintain the docket, or log the seizure, is a misdemeanor offense for the chief of the department and the officer making the seizure, respectively. This statute was passed due to a number of cases where a firearm was found in a vehicle, the officer asked who it belonged to, received a reply of, "It's not mine," the officer seized the firearm, and announced that the owner could claim it at the station. In some cases, the agency and/or the officer later obtained legal possession of the firearm as it remained unclaimed/owner could not be determined.
I don't consider the temporary custody of a firearm during a traffic stop to be a seizure. I have disarmed citizens during routine traffic stops when I have visually discovered a firearm. I returned the firearm, unloaded, at the end of the traffic stop (I also returned the ammunition).
The only time I have ever asked if the person(s) have a weapon in the car, is when I have determined that someone is going to be taken into custody. Then I want to insure that the weapon is under my control during the arrest, transport, and booking process. Since there is an arrest, there is a legal seizure of a firearm.