Comment by schroeding

You cannot prove the absence of e.g. a Veracrypt hidden volume or similar, though. Even if you honestly give up your key, you could still be either

A) held in contempt of court, if the authorities do not find what they expect for some reason and accuse you of using such techniques or

B) if you specify that such behaviour by law enforcement is overreach, have a clean way out for criminals, codified in law, heavily damaging the impact you may expect of such a law.