I've been meaning to ask this for a while about my 2004 L300 Wagon.
The car will beep/ding for a number of known reasons because it's usually accompanied by a dashboard light.
There are a couple of times I can't figure out.
1) I've stopped the car while waiting for gas, restart it to move up and sometimes I'll get a single beep. No low fuel or any other dash lights.
2) When I stop the car to get out, using the lock all button on the door will sometimes generate beeps, other times not. What's the difference?
Trivial but annoying.

 

I get a beep at the service station line also, I think it is a seat belt reminder but no message. I do not get a beep ever with the door locks though. I have a set of manuals for the 2000-2002 cars, the later cars have some different systems.

 

I have a 2004 Ion, and with the engine off, key out ot the ignition switch, when I press the door lock button (to lock), I get a chime but the locks do not activate. I have to press the lock button again for the doors to lock (no chime this time). I think the first chime is just to have you confirm you want to lock all the doors by pressing the lock button again. I'm guessing it's just a safety thing so you don't 'accidentally' lock yourself out.