Two weeks ago I parked the car and all was fine. Then 6 days later I find the battery (6 months old) totally flat (2.5 V across the terminals). I jump started the car and measured the current with a clamp meter: 30 A charge going into the battery. Voltage OK. All seems well, perhaps the missing rod on the rear hatch operated interior light switch caused it to drain the battery (but I was convinced I had put the switch on the light itself in the off-position)...

A week later, all is still fine.

Another week later, all is still fine when I leave for a 100 km (62 mile) trip. When I return, in the last couple of miles I suddenly notice the charge light come on dimly, and so do the two to the right of it(handbrake and exclamation mark).
While driving I plug a simple three LED charge tester in the cigarette lighter, and sure enough, it shows low charge.

A couple of minutes later I see the charge light flicker and suddenly go off, and the charge tester shows a normal charge. It didn't come back.
At home I test the voltage with an idling engine at the battery terminals: 14 V and at the alternator: 14.08 V. Looks okay to me.

I'm not entirely sure that the flat battery two weeks earlier and this charge light glowing dimly are related, but who knows?

If not, is this the first sign that the brushes on the alternator need replacement, or do I need to budget for a new alternator?