2009 Prius - Amber Brake Light in cold weather

nah, it was probably an anomoly. nothing to worry about unless it comes on again. welcome to priuschat, all the best!

 

Which light is that: ((!)) or ABS or VSC ?

If one of those lights came on, and remains on, then you should have it checked out. No known issue associated with cold weather.

 

I'm guessing you didn't go below zero, but below freezing. In the Seattle area we made it down to 18.

I'll wager the light you saw was looked like this (!)

If so, that is the car telling you one (or more) of the tire pressures is low. It is normal for pressure to drop as the temperature goes down.

 

While my home near Seattle dipped only to 16, I spent most of the cold snap with family elsewhere in the Pacific Northwest. I saw zero-F both along the route, and at dad's house. Working a bunch of livestock, it was a challenge to keep the veterinary syringes and needles from freezing. When the porch temperature rose to 14F yesterday morning, as I was leaving, it felt like a very pleasant warmup. Numerous surrounding areas had gone sub-zero.

If the tire pressure light comes on due to cold, there is a good chance the tires are somewhat low even at normal temperatures. Check them.