I had the same issue over the last couple weeks. Car would start fine if i drove it every day, had to put a trickle charger on it if it sat for over 3 days. Its been discharged several times in the last few years.
Checked voltage resting 12.3v charges at 13.7 with car running. New battery is on its way, 3 months out of warranty. I need to fix dome lights windows roll up with door open.

911obgyn -- took me around 15 minutes to replace my driver's side switch, be sure not to let the wire attached to the switch drop back behind switch once you've pulled switch out of door jam. Replacing my pin switch both allowed dome light to turn on/off when door opened AND eliminated my battery drain issue

AGM batteries reguire a higher charging voltage than a regular lead-acid battery.
Late vehicles' alternators (14V+) are designed for use with AGM batteries not early 911s.
Avoid using AGMs, if the vehicle isn't designed for its use!

Good theory worth exploring

I don't drive it much the last 2 years.
Always on tender.

Update:

Left it off the tender the last time I drove it.
Tender could not revive.
I hooked up a 12v charger overnight

Battery now showed 12.65v at rest
14.05v while running

Took it to FLAPS
He attached a device that read 850
(Battery is only rated 765 CCA)
Device said green/OK.

Took it home
Put in car
Went for a drive

Left it off tender
I will see if the car starts on Saturday (3 days)