Yes,I've read that too, but can't give chapter and verse. It may have been some author's speculation or a deduction or it may be on express authority, or it may be just myth.
It does make a certain sense. Customs are really supposed to act on reasonable suspicion.In any event, customs have always had greater powers, more draconian powers, than other bodies . When police arrest somebody on reasonable suspicion and that person is deprived of his liberty pending investigation, he has no redress in practice. In fact, if he's charged, tried and the case is thrown out by a judge , he doesn't get proper redress e g for loss of earnings .Putting those together, I can well see that customs would just answer 'Tough!' and be entitled to leave the person the job of reinstating the car. What stops them is that it looks bad, whatever their rights in law|The press would have a field day in most cases and customs would lose whatever public sympathy they have.