Without pressing the "Power" button, the car will stay on until it runs out of gasoline. The Smart Key System does know that the key has left the car, but all it'll do is beep loudly at you complaining that the key has left and the car is on. It also will not let you lock the driver's door remotely while the car is still on (it'll beep and complain, and unlock the door). Anyone can get into your car and drive it off, and you are wasting gasoline and the emissions could be dangerous in an enclosed garage...
Follow your owner's manual procedures for turning off your car: put the car into park (P button), apply the parking/emergency brake, and press the Power button.
However, if you do not press the P button to park, the car will "press" it for you when you press the Power button to turn the car off. The parking/emergency brake use is also voluntary - it just prevents a little roll until the car settles against the P parking pawl in the transmission.
You can learn more about the Smart Key System through the flash demo at:
http://www.toyota.com/prius-hybrid/demos.html