It's not theft to take something when you genuinely believe that the owner wouldn't object to you doing so. (It's immaterial as to whether the owner actually would mind or not). The Theft Act 1968 states:
"A person’s appropriation of property belonging to another is not to be regarded as dishonest . . . if he appropriates the property in the belief that he would have the other’s consent if the other knew of the appropriation and the circumstances of it".
However it's unlikely that any court would uphold a claim for damages against you if the car driver (or someone he gave/sold the scooter to) suffered any injury from using the scooter. (A court would almost certainly rule that, given the circumstances under which the scooter was acquired, it was up to the person who took it to check it for safety).
So 'the law', per se, isn't directly relevant here. If, through your public spiritedness, you want to try to warn the guy about the condition of the scooter, you're welcome to try but the police would almost certainly refuse to get involved (because no offence has been committed) so you'd find it difficult to trace the car. It's possible, under limited circumstances, to find out the registered owner of a vehicle through the DVLA. (That's how private car park operators pursue people who don't pay for their parking). But I doubt that the circumstances you refer to would satisfy the DVLA's requirements for providing the information.
Chris