This phrase doesn't sit well with me. They cannot establish the existence of evidence without having found it. They can't find it without a proper search. They can't execute a proper search with a warrant or reasonable suspicion.
The existence of evidence is established once it's found and documented.
Edited to add: if they can arbitrarily establish the existence of evidence, and then don't find it, then "obviously" the suspect is a no good dirty rotten evidence destroyer.
It's not arbitrary. It depends on other pieces of legal proof, like statements from witnesses, other material evidence, etc. For example in case of hidden partitions people usually get busted because forensics find data directly referring to it (eg. in caches, metadata, logs, thumbnails).
The judge has the power to let them search your property.
Nobody has the power to let them search your mind, and they can only do so by imprisoning you or otherwise depriving you of your rights.
The distinction between a search warrant and self incrimination is a big one. You should always have the right to say nothing.
If forensics and data know for sure you did something, the stuff in your mind is unnecessary. Either they have the weapon or not, so to speak. You shouldnt be placed in a position where the justice department "is entitled to" the knowledge in your head.
Warrants dont establish existence of evidence. they just authorize the state to override your rights, because they convinced a judge. All of this is weaponizable.
The way to establish evidence is to have evidence. If you dont have evidence you haven't established it. Forcing you to divulge something in your head is to force you to give up evidence against yourself, and the ultimate trespass.
I can't think of a situation I'd authorize someone else to do that.
I'm not agreeing with the practice, I just explained it, how pieces of evidence (and testimony) can legally establish that there's evidence that the defendant has ways to produce.
I dont want to live in your society. I don't want to empower anyone to do that to me, even if they claim they're only going to do it to criminals and bad guys.
The terms of service change.
Of course, where the line is blurry, is as you rightfully note, beyond a point no justice system can be certain that the defendant really still has access to whatever evidence, be it some part of their mind's contents (so a memory, the location of a body, a password, a face), and even if somehow we know they had access but now they don't it'll be impossible to determine intent. (So destruction of evidence cannot really stand when it comes to forgetting things. Unless, maybe if the defendant caused brain damage to themselves intentionally.)