The buzzer that sounds when you leave the car and have left the lights on has not been working on my 02 plate Honda Civic for at least a year. I've tried to find the buzzer on the circuit diagrams I have on CD, but to no avail, and was reminded that I really must do something about it after finding a flat battery one morning a couple of days ago.
A quick Google didn't show up much information on how to replace/find/diagnose the buzzer, but one webpage said that sometimes the drivers door switch can fail. So this morning I took a look. First indication of a fault was that the drivers side courtesy light doesn't come on when the door is opened - even after a year, I hadn't noticed this - no idea why. This is a smoking gun pointing at the switch in the door, so after pulling off a cover from the screw, I removed the switch from the door jamb. There was obviously a problem, because the switch itself wasn't firmly attached to the rubberised carrier, so I reattached it, with a satisfying click, remounted the switch in the door, and the interior light came on. Hurrah. Tried the lights with the door open - and the buzzer noise filled the air like a Beethoven concert. Well, perhaps not that good, but satisfying nevertheless! Although the swtich was still working, because it had come lose from its carrier, it wasn't earthing to the chassis, which was requried as it there was only a single power wire to the switch and no return except via the chassis.
So, a year or more of occasional flat batteries was fixed by three minutes work with a screwdriver.