When I was in the car business, a "pilot" car was one of several kinds of "pre-production" cars (mules, prototypes, alpha build, beta builds, etc. ).
Like Dobalovr said, manufacturers of course build a few (several hundred sometimes) pre-production cars at various stages in the product development process to debug the process, to crash test, and a litany of other engineering verification/quality control steps before "turning on the assembly lines"
Anything OTHER than a "production" car -- a vehicle that's been through ALL the pre-production steps and is LEGAL TO SELL (Street Legal) to the public -- we wanted to FIND and DESTROY.
A few might have been around one of our technical centers/with our suppliers somewhere, or donated to a university/school for vocational education, etc. ..but we knew where EVERY ONE of them was.
People (always a "company", never an "individual") outside our company who had them had to (1) return them to us on demand, or (2) independently verify their destruction (sometimes under our supervision depending on the car) to us.
The key thing used to be was whether the vehicle legal to sell to/be used by the public (again, this is a legal/engineering definition). Some "pre-production" cars were designated as such (e.g, cars we let a few fleet customers drive around for us under special arrangements), but most were NOT "street legal".
All this was as of 8 years ago...I dunno the state of affairs today. I believe it is still the case that vehicles that are NOT legal to sell to the public, now or 40 years ago, should NOT be in the hands of private citizens.
Again like Dobalovr said, I defer to expert knowledge on this topic.