Well it looks like my Prius C took an hybrid vacations and it runs only on gas. This started today. I turned off the engine, remove the key, inserted the key again, it says "Ready" and then after 4 or 5 seconds the gas engine starts. Another weird thing is the battery meter indicator got stuck on 2 bars over the middle.
I found the problem, had a plastic bag behind the seat and was obstructing the fan from the battery. I removed the plastic bag and everything returned to normal.
Wow that's cool to know there's a sensor that knows if the battery fan is being blocked. I have a habit of putting small parcels [groceries, etc.] behind my seat in the footwell, and so far I've been cognizant of the fan inlet.
It's nice that you found the bag blocking the vent, but what you described above sounds completely normal or you didn't clearly explain what is abnormal.
Where is the vent for the battery? This is something I've never heard of before and my dealer never mentioned it. I just bought by Prius C about a month ago, and after we took it up a pretty steep drive into the mountains (I live in Utah) last weekend it seems to be acting differently. The secondary engine (the one you hear when you accelerate) sometimes comes on at very slow speeds and even when sitting but the battery is still fully charged and this never happened before the trip to the mountains.