Here is my story of a dead 12v battery. Last Saturday, went to the drive my 2018 Model 3 in the morning and nothing worked! Only the front passenger door was able to be opened. The car was plugged in at home and couldn't release the charging cable. Looked online (youtube and forums) about my symptoms and found the whole dead battery issue. Called AAA and got a battery tech over to see what they can do WITHOUT stating it was our Tesla. To my surprise when he arrived, they didn't service or replace Tesla batteries! He left did more research and I couldn't purchase the same battery anywhere! Called Tesla roadside assistance and they said it would have to be towed to a Service Center at my expense since it was out of warranty, but it did sound like a battery issue. The tech on the phone said it could be the battery, but it could be something else. Anyways all service centers close to my location in Southern California were closed on Saturday.
I next tried to jump start with a jump-starter from Costco. One of those small battery packs that I've used on my other ICE vehicles. That didn't work. So, I went back online and found videos showing how to replace the battery. I took the 12v battery out, after disconnecting the HV battery connector, and connected it to a trickle charger. I left it overnight and reconnected my now $60k paperweight and everything worked!!! This was Sunday. Monday, I drove it to the Irvine service center and they said they could look at it and give me a $100 Uber credit to get to where i needed to go. I asked it they would just sell me a new battery so I could install it myself and they said "yes" and charged me $91 for the brand new battery. Now I have a fully charged old battery and a new one in my M3.
Here's my question to you. How many miles were driven when your 12v battery died in your Model 3? My M3 battery died at roughly 59,800 miles. I had no error message about my battery as well. Sorry for my long winded story!