Only time i've seen a genuine test vehicle was a BA XR6T test car, it had a basic interior (Like bottom like everything, no carpets, no trims, a dash but no glove box/etc, plus lots and lots of holes drilled everywhere where they obviously had things mounted).
It also had a VIN that was basically the ford code and then all 1's.
The car couldn't be registered because the vin was an invalid VIN and the car wasn't ADR compliant. As such the car was being used as a track car at the time.
Even the
ECU was a pre-production
ecu that was so wonky and locked down that it couldn't be accessed tuned (by sct xcal at the time). After much mucking about, they ended up getting a
ECU and engine harness (as the harness with the test-bed
ECU was very different to production) so they could tune it.
From memory that introduced a myrid of other bugs in the car (due to pre/post production mixing) but nothing that really effected it in the sense of tracking the car.
It didn't have any special plates, just had all standard ford stickers and such and the VIN was invalid.