How do you scan for a stealth RFID?
Quote from SR4A page 329:
Stealth tags are encoded to remain silent and only
respond to transmissions with the proper passcodes. They cannot be
located with a commlink or bug scanner (unless the codes are known).
They typically use special frequencies and other tricks to avoid detec-
tion.
Most of the time it doesn't send ("remain silent"), if it sends it does so on "special frequencies" and it explicitly says, that comlinks and bug scanners (I will interpret this as meaning radio signal scanner, since I don't know of any bug scanner in the Gear section) cannot locate it.
You can't scan something that doesn't send and if it does send, it says you can't locate it.
Let's assume one can at least notice, that something was sending even if you can't say from where exactly it does so.
Then you could work with the following procedure:
1. Put the stolen vehicle in a faraday cage.
2. Use EMP Grenades or fire with the HERF-gun (after defining with your GM how the beam of the gun is formed) until you think you had the whole vehicle covered.
3. Scan for signals for a long time like day or two to see if something is sending from the vehicle
4. If there is anything left sending in the vehicle, repeat steps 2. and 3.
After this you can be reasonably sure to have catched all stealth chips that have a mean sending interval of a day or two. Bad luck if the owner or the corp which build the vehicle has put into it a stealth chip which only sends a signal once a week.
The only other method would be to complete dismantle the vehicle, throw away everything which cannot be divided into smaller parts than one centimeter thickness, let a drone go over it centimeter by centimeter with a tag eraser and put it together again (after buying the parts like an engine that cannot be disassemble into small enough parts)
That would be RAW. Don't know if riggers will like it.