An awesome thing about a Prius is its ability to heat and cool you all night while only running the engine a fraction of the time and using a tiny bit of fuel. I have very much enjoyed using it this way and expect to do more of it. And I have very little worry about CO, because of the engine's low emissions and also how little of the time it will actually be running. But a CO detector/alarm inside the car is very cheap insurance. I prefer the kind with a numeric parts-per-million display rather than just a honker. I bought one of those for not much money, and from inside my house with the windows closed, it was able to detect that the guy across the street was having his carpets cleaned. (The carpet-cleaner truck was parked in front of his house, running its engine.) This article today reports three deaths and two hospitalizations of people inside a travel trailer in a campground, apparently overcome by CO from "a generator that was located near the travel trailer" (doesn't say whether it was their generator, or one from a neighboring camper, or how big a generator). Where the CO goes, and whether it accumulates or not, can depend in weird ways on what the wind is doing and the terrain and surroundings and such, whether it's CO from your own exhaust or something else nearby. While I think it is hardly ever an issue while Prius camping, an inexpensive detector can make sure you notice in those rare cases when it might be.