I had an intermittent indication recently.

I found that the left front marker lamp was not on when the fault light was on. I bought a new bulb, and waited. It mysteriously repaired itself at least 10 times.

On the eleventh try, it failed and didn't repair itself, as the fault light stayed on. I finally replaced the bulb and no more fault.

When I looked at the filament, it was indeed broken from one holder. My guess, it broke from the bottom and as it dangled and jiggled around, it sometimes contacted the rest of the filament and temporarily welded itself back together. Sometimes it would happen once or twice over a bumpy stretch of road.

As far as I know, it is indeed for outside lights. Since the switch was off, I'd suspect a brake filament, since unless you have a helper step on the brake pedal, you won't see a bad filament outside yourself.

Corrosion, as mentioned, is possible, as is a bad filament. Pull 'em, check 'em, clean 'em, and replace 'em if in doubt.

Best Regards,
Jim