Flying Lawyer:
If what they say may in due course be used against them in a criminal court, then IMHO they must be warned. That’s a well-established principle of the legal systems of most (all?) developed countries.
Out of curiosity... would that then mean that a reporter, for instance, would also have to give warning to an interviewee who was about to implicate his/herself? If a person were to write a tell-all book about their life as a criminal who had never been caught, giving details of time, place, event, etc., would that be admissible in court even though the author had not been warned that this information could be used against them? If I make an admission to a crime on a public website, isn't that admissible in court as evidence?
I'm not asking these questions to be difficult or anything... just trying to understand exactly how far the obligation to give warning extends exactly.