I just came across a great hybrid (depending on the exact use case): The Automatic Pro plugs into a vehicles OBD-II port, transmits location updates every 20 seconds when the vehicle is in motion, and draws its power from the car.
Yes, it's an "active" tracker, but the $130 purchase price includes "at least" 5 years of 3G data service.
So it's not exactly what was specified: It's somewhat traceable, it requires access to the inside of the vehicle to place, and it would be readily noticed by anybody checking the vehicle's OBD port. But on the plus side it offers real-time tracking, IFTTT, and other integration and safety features.