Actual "undercoating" was a old dealer high profit add-on to the price of a new car. Newer factory paint processes includes everything that's needed from the factory. I assume you have a light color Prius which looks like something is missing. I'd feel comfortable that you're Prius was properly painted.
Effective undercoating was NEVER offered by dealers excepting for a high profit "add on" at the expense of the car owner. Proper "rustproofing" first and then undercoating is still done for commercial, military, and industrial equipment. When done properly, it is very effective.
