If this fades out after a while, it is likely the phosphor on the white LEDs glowing (similar to how a glow in the dark toy will fade out over time after it stops being exposed to light). White LEDs are actually blue or ultraviolet LEDs with a rare earth phosphor coating that converts some blue photons to yellow photons as they leave the LED’s built in optics.
If it doesn’t dim/fade out after time then it is likely that the circuit is still energized (but off) and that you’re seeing the effect of leakage current going through the LEDs...
I noticed this LED glowing several times as well (new Model S September 2018), it even stopped after a while and suddenly started glowing again after the parked car made some 'getting a live' noises. First of all I was told the car was getting ready to receive the Version 9,0 update but this did not stop it... . Now I'm told this is a known issue for all cars from chassis number 180000 onward. The cause seems to be capacitors loosing their energy but it would not being of influence on the battery.
What is strange dough is that once it is off it suddenly starts up again so the capacitors seem to recharge all the time ...
I posted about this months ago. When I brought it up with the SC they just kinda shrugged their collective shoulders and said it's a software problem. It's definitely not an afterglow or charged cap sort of issue as it will continue this way over night (8+ hours).
Never happens on our 2015, occurres on 2017 and on 2018. My guess is Tesla left the headlight LED controller circuit on some shared voltage rail which doesn't turn off (or can't be turned off because the same rail powers something which needs to be ON), but the LED controller doesn't cut the current down to 0. The LED headlights are a more recent design. Tesla has been cutting corners for last few years, not doing as solid of a design as the pre 2016 cars.
I see this on my December 2017 build. In fact, I was pretty used to seeing it all the time when my car was plugged in at home and I was going into a dark garage.
Last night was the first night in who knows how long where I specifically noticed it NOT happening. I was so conditioned to it, I was actually worried something was wrong when I didn't see the afterglow.