Toyota Dealership left my Prius ON in the lot

I strongly disagree. With modern immobilizer systems modern cars are *very* difficult to actually steal (unless you already have left it running for the thief).
Most break ins will result in theft of valuables, air bags, etc. from the car but not in theft of the car itself.

 

That may be true for a vehicle with a traditional key, but the Prius can't be hotwired. For this reason, the Prius is broken in far more frequently than stolen as whole (usually towed away).

 

I assume you would have to leave the remote inside the car, otherwise it would beep continuously... If the thief breaks the window to open the door, couldn't he simply drive away? Big drawback?

 

I take the keyfob with me. If someone does get in past the locked doors, they could drive off. Once they turn the car off, there's no way to start it again without the keyfob.

It comes down to a judgement call about the relative dangers. Can I risk someone breaking a window to steal the car in order to have the A/C on while I'm shopping? Returning to a comfortable temperature car instead of an oven is really nice in hot weather. The electric A/C and the hybrid setup is what makes this possible with the Prius. The ability to leave the car in READY mode without leaving the key is also important.

I try to avoid even visiting areas where I'd be afraid of someone breaking a window. The idea of some lowlife actually wanting to steal a Prius strikes me as remote...