I just came off an extended bout (3 months) with the Tesla SC where, shortly after upgrading my 2017 MX from AP2/MCU1 to AP2HW3/MCU2 the 12 volt battery started dying every couple of weeks.
The experience forced me to re-evaluate what apps/accessories I wanted to have running and the impact they have on the car.
I love TeslaFi and have been using it since the day I picked up my car it is easily the best tool I've used to know everything about the car. With a LOT of testing I found:
1) Before the Beta "Polling Delay After Waking Up" setting - TeslaFi was absolutely keeping the car awake when not plugged in . Updating this to a "1" for me resolved that issue. The car - for the first time since I have owned it - now sleeps for many hours at a time. Typically waking only when I do something to wake it up. Given that I've monitored the daily drain with and without TeslaFi, I don't see any difference when the car is plugged in if TeslaFi is monitoring or not.
2) As others have found, my car cycles when plugged in (11 minutes sleep / 46 Minutes idle / rinse & repeat). This leads to a battery drain of about 2-3% per day.
3) I have my car set to charge beginning at 1:00 AM to take advantage of EV rates, so I don't have the same concern as some that the car is incrementally charging all of the time. But I agree that the constant cycling is a worry and am now making it a point to unplug it and leave it unplugged when the SOC is something approaching reasonable (i.e. > 50%)
4) Side-note about Sentry mode for completeness as this has probably been done to death elsewhere, but I watched this carefully while evaluating everything else and, with sentry mode active and my home excluded, energy consumption jumps to between 4-6% per day. I can imagine using it briefly for parking in and around the city but it's absolutely useless for what I'd consider the most relevant use which would be parking the car in a long-term parking lot while traveling by plane or ship.
I'm currently sitting with firmware 2020.44 and waiting for the amazing upgrade that will stop the cycling. But at least, since the car is plugged in while it's waking up constantly, I don't have to worry about coming to my car and finding it's SOC uncomfortably depleted. It will top off once a night and start cycling immediately.