Will ask my mate who used to work for them a while back about unmarked cars and see what he has to say. There would be nothing stopping them from collecting information I wouldn't think, but having the powers to pull you over and fine you would be a different matter I would think in an unmarked car. Only unmarked cars I've ever noticed with flashing lights installed have been the cops.Please someone get a license plate and car make and model etc.
I do not believe undercover work is legal for them, the police have special provisions for undercover and unmarked cars, there is nothing I have seen for the DTMR to do it. A private investigator might be different though.