Hundreds of online forums going back more than a decade reflect that there is much confusion around the significance of the stages of locking a car with a remote. With most newer remote locking systems, the first press of the key fob will cause the locks to engage and security to be armed.
Pressing the lock button a second time will cause flashing lights or an acoustic alert as a psychological confirmation. But many people assume that the first press of the lock button locks the doors and the second press arms the anti-theft device, and think that the "confirmation" sound is related to the anti-theft system, when it isn't. With most anti-theft systems, going back a quarter of a century, and including pre-factory installed aftermarket car alarms, the anti-theft system arms itself after a brief interval after the door is closed, whether or not one presses the key fob.
This passage appears in the 2015 Chevy Impala owner manual, but phrasing like this will or should appear in most current owner manuals:
This vehicle has a passive
The system does not have to be
manually armed or disarmed.
The vehicle is automatically
immobilized when the key is
removed from the ignition.
- Section 2-18, Keys, Doors, and Windows
In older cars, a button on the dashboard often indicates that the alarm is armed. Today most remote locking systems use a two-step approach, or offer a means of switching to a visual alert, or both.