The police go to the judge with a list of the guns they think you own and the judge will tell you to turn them in or be held in contempt.
Not sure how they can hold you if you don't possess guns here and aren't otherwise a prohibited person. Further, by this reasoning, there's going to be a lot of people who cannot produce guns that they've sold a long time ago etcetera that will appear on the stupid list... So they can hold you indefinitely for not producing guns you don't physically have? Not buying that. I've heard of people being threatened with this but I've never heard of it actually happening. I think 99% of it is a big lie used by the PD to try to get "more" stuff out of people.