So I finally got this taken care of. It was a tremendous amount of frustration and effort for what was just a dead battery. The subaru dealership wanted the entire car to be brought in, not just the battery. Subaru roadside assistance sent the wrong tow truck twice (our cars need wheel lifts and dollies). I did learn that keyless entry is nice until your battery dies and locks the steering column with it.
Finally the dealership sent a tech out with a new battery and we got the battery swapped and then drove the car along with both batteries to the dealership for testing.
At first they wanted to blame the dead battery on my audio amp. A quick test and some easy deduction showed that not to be the case. However, they were still stumped because the parasitic draw was over 100mA while most other cars in the shop (imprezas, forester, etc.) were around 30mA.
They called Subaru up. Subaru said we don't have a spec for the BRZ but pointed the tech to a published guideline of less than 70mA. After enough fuse pulling they tested a showroom BRZ and it was drawing 150mA. They deemed my 120mA to be "under spec" according to this and ended up replacing the battery.
The lesson here is our cars seem to draw significantly more current than other vehicles while off. If you're going to leave it for the winter, it's best to put the battery on a float charger. I now have a pigtail running out the hood into the garage to keep things topped off.
Here's the official write up.
