I suspect it's a fault indicator for the BCM (Body Control Module).
Suspect that any fault it detects will result with dinging... and it doesn't necessarily wait until a door is open or a key is near.
I've witnessed the periodic dinging without everything shut off, locked and no key around.
I had switched over to LED and it consistently dinged the entire time. Could be coincidence, but the dinging stopped immediately after installing the resistors a week or two later.
BCM controls and interfaces with a lot, as you can see in this graphic...
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