Re: Under the category of strange stuff...
I wonder if the airbag codes "sound" in your year rig? In my '97, the airbag codes just flashed on the dashboard, but they did so in two bursts of different numbers (for example, three flashes, then six flashes was code 36). This sequence would repeat, IIRC, five times then the airbag light would go solid "on" and that was that until you fixed the problem.
Now, this doesn't totally figure for your thing, since apparently your airbag light did not come on. But it still somewhat reminded me of that with the two-number sequence, and who knows how that same function might have evolved. BTW, the airbag code numbers were a whole difference universe from engine codes. I found all this out when mine started flashing - turned out to be the connector at the (swivel) passenger seat.
Anyway, no idea, but just a few thoughts.