Mine are dead or hibernating too.
I've been filling up at the same Shell station with few exceptions since sometime last summer, always with 91.
Through the warm summer temps, cooling off fall, and even the cold snap in early winter when it was low 20's/high teens in the morning (I'm in SoCal, that's Arctic weather for us!) it was chirping away.
I've noticed since the warmer temps we had in January it hasn't chirped.
Now that I said that, I'm sure it'll chirp this morning.
