Believe me, I know this sounds crazy - but on random occasions I have some kind of melody coming from under my dash. It's three notes descending from a high note, middle, then a lower note. It will randomly repeat several times, goes away for days or weeks, then reappears.

It is not the radio, door ajar, or any other common audible warnings. The best anyone can tell (because it is not very loud) is that it's coming from somewhere under the dash or behind the area of the climate controls. It's kind of faint, not very loud at all, so it can't be any kind of warning sound. It only does it on start up but does not occur more than a minute or so after startup. Happens with the doors open or closed. It is not the aftermarket radio, as it does it with the radio removed, and the sound does not come from the speakers, only from somewhere under the dash.

I saw one reference to this problem here on the forum, but it was a ten year old post and there was no resolution to it.

Anybody have an idea of what the hell this could be?

My first guess would be an after-market alarm system that may of even been installed by the original dealer at time of sale for that car.

Maybe look under the dash for any odd-branded boxes?

Piggybacking on the aftermarket suggestion...

Hints that may help: Do you use a key fob other than the OEM Toyota fob? Is there a flashing LED somewhere on your dash or windshield that does not look OEM?

Sounds like a "Buy Here, Pay Here" tracking/Disabling system has been installed. If you just turn the key to the run position before cranking, does it make the sounds?