Beeping noise when opening door?

[minervamoon]

Maybe I'm just being too picky, but is the driver's side door supposed to beep when I open up the car?

To unlock the car, I usually use remote keyless entry. If I hit it thrice to unlock all the doors*, and then open up the passenger's door, it does it peaceably. But if I hit it once and open up the driver's side door, it makes one continuous beeping noise (the same as the headlight noise when you stop the car and open the door without turning off the lamps, except they're not even on) that lasts for 20 seconds or until I close the door, whichever comes first.

It seems, moreover, that if I unlock the door and don't actually open it until about half a minute later, the beeping noise doesn't happen.

Is there some kind of Toyota failsafe that wants you to know, despite the fact that you've just opened the door yourself, that the door is, by the way, open? I've tried hitting buttons, flicking levers, moving switches, etc. while the beeping noise is going on, but nothing ever seems to quell it.

*which doesn't always work, but that's another matter altogether