'When you remove a power box from a vehicle, is it still possible to tell whether it was ever installed? Does the box leave any data in the vehicle's control unit? '
If the device was set to an extremely high level or was poorly designed, error messages might remain.
But even those will have their 50 engine starts since the last appearance (or after the removal of the gearbox) deleted - if there are no other errors.
This applies to the VP engines (90 and 110 horsepower). I don't know if PD engines behave differently.