For the last several days I'd noticed the auto idle stop not working. Regardless of the time of day I drove or type of driving I was doing it didn't work. I didn't have the A/C on at any time, and the temps had been in the 40's.

Well I read the owners manual and found out that if the car is in a defrost mode of any kind the idle stop will not work. In my case I had it set to warm the feet and the windshield, but I didn't have the A/C on. Once I switched to floor only the idle stop magically re-appeared.

This is different then the Prius and caught me off guard.

So FYI and read your owners manual!

 

the A/C is probably engaged in defrost mode to remove humidity from the cabin and prevent condensation. Even though you don't have AC on it is probably on every time you hit defrost.

 

You are probably right. Even though it clearly said "A/C off" on the control panel.