Be careful about depending on that. We've had a few technicians (not me) lock themselves out of cars because the battery in the fob was worn and got shifted to another part of the car before the 'lock and leave.'
Most of the time you'll get a "battery low" warning when you shut down the car with a low fob, but we've had some peeps get gear out of the cargo bay and accidentally leave the fob there....only to do a lock-n-leave. Reason #2653 to tie-wrap a keylet to an exterior hard point and hide a (debatteried) fob inside the car. Good Luck!
Some years ago, the "Hemlock Society" said that connecting a hose to your tailpipe and breathing is no longer a reliable means of passing away. The reason is the catalytic converter. Nonetheless, the cars do emit some carbon monoxide which I imagine can build up in the garage and then invade living space. I personally find the piercing alarm a good reminder to turn off the car -- sometimes I forget because the car is so quiet when it is parked -- but perhaps this isn't adequate for others.
It would not be a bad thing to install a CO detector in your garage as well as having the Prius beep a few more times when you step out of it and close the door.