One thing that has helped me with stealth camping in the Prius is the rear window coverings and the front window sunshades. The rear window Shade Sox allows me to keep the windows down while sleeping for ventilation and allows me to hear anything outside the car without anyone outside able to tell that the window is down.
As you can see in the image below, you cannot see into the vehicle at all. The front window sunshade is easy to set up in the car, combined with the custom fit window sunshade little to no heat from the sun enters the vehicle.
See the links below for the products I am currently using on the Toyota Prius for Stealth Camping.