It may be that water/moisture has gotten in behind the door panels...do you notice that it worsens in damp(er) weather? The car unlocks by itself if you leave the key
in the ignition and you try and lock
Or somebody has the same coded fob (unlikely).
You might want to reprogram the remotes if you determine that the area behind the door panels is dry and there are no corroded wires, etc.
2004 Nissan Quest Remote Programming
Note: You must have all remotes on hand when performing this procedure. Any remotes not used during this procedure will become inactive.
1. Close and lock all doors with the driver's side power lock switch located on drivers door panel.
2. Insert key into ignition (DON'T TURN) and remove it from the ignition key cylinder at least 6 times within 10 seconds (this will need to be done at approximately one second intervals). You may need to insert and remove the key up to 10 or 12 times until your hazard lamps flash. PLEASE NOTE: Withdraw key completely from ignition cylinder each time. If this procedure is performed too fast or too slow the system will not enter programming mode.
3. Insert key into ignition and turn to the ACC position.
4. Within 5 seconds, push the LOCK button on the keyless remote. Your hazard lamps will flash. PLEASE NOTE: Do not press the button on the remote more than one time or the programming will not be successful.
5. If there are any remaining remotes (including working ones), Unlock then Lock all doors using the switch on drivers door panel. Within 5 seconds of locking and unlocking doors press LOCK button on the next remote. Your hazard lamps will flash. Repeat this step for each remote (including any working remotes).
6. When all remotes have been programmed turn the key to the OFF position, remove keys from the ignition, unlock doors using the driver's side power lock/unlock switch and open the driver side door. Test all remotes.