I've reported this issue before, both in the TMC forum and to the local Tesla service shop. I have an update.
First, the problem, or, mystery.
Lately, as in past few months, most mornings when I come down to the garage, unplug the UMC from the charge port, and manually close the charge port door, it makes no electromagnetic clanging sound to indicate it's locked. It usually does when I am on the road somewhere supercharging, and when I've tried to reproduce it at the local Tesla Service Center, it always loudly clicks right on cue, but when I am at home, usually not.
What I also noticed lately, as in since around December, is that when I get in the car, which says "Car Off", and press the brake pedal, the pedal is very stiff, and requires some effort to push it down for the car to start. Right when I do this, I hear a loud clapping sound in the rear portion of the car, and suddenly the car wakes up.
It was only this morning that I figured it out --- it's the charge port door, finally locking. That's what's making the loud clapping/clanging sound.
Questions: does anyone else's car do this? Why has my car started to do this? Is this a symptom of a problem, or just some harmless weirdness?