Or say authorities demanded access to a long lost forum account you haven't used nor remember the password from many years ago. Can they use this to detain people as a loophole... just find accounts you don't use anymore and demand access to all. Seems a dangerous law to allow.
This is part of why democratic norms are as important as law: it norms as a kind of cartilage leaving the law to be relatively simple and strong. If you lose the cartilage, it's painful bone-on-bone (that is, you are forced to try to pass unambigous laws with norms baked in which tries to replace norms with laws, and the downside is in a complex, slow system, which is it's own sort of peculiar horrible injustice).
There are too many examples in the US and Australia where LEOs ignore all norms, seeing them as an unjust limit on just power, and manipulate the (asymmetrically well-known) law to retroactively justify any action. Even when LEOs are legally challenged (a rare enough act of courage, and risks further victimization) and time and again get away with it anyway.
And yet, the people who should be clamoring for stricter, fairer justice against LEOs are LEOs, because every time they get away with it, they win the battle but lose the war for legitimacy. When LEOs lose legitimacy, it leads to a cynical world where laws like this password one are expected to be pushed to their absolute limit by LEOs, and it will be, once again, up to the courts to enforce the norm.
I fear that the modern smartphone is just too big of a temptation for centralized authority, because of their universal appeal and potential for as close to a (retroactive AND realtime!) panopticon as we can get without the palantírs from the Tolkien universe.
Why don't we force people to put cameras in every room of their home with full access given to the law enforcement? And if you resist this, do you have something to hide? I think it's interesting how strong this argument is, even in it's most obviously oppressive form. Coming up with an equally satisfying counter-argument is our #1 priority as privacy advocates.
The law is capable of taking into account probabilities.
Claim to have forgotten the password to a forum that you haven't used in a very long time? That's quite believable.
Claim to have forgotten the password to, say, your password manager that you successfully have typed from memory several times a day every day for the last 10 years? That's a lot less believable.
I reckon authorities have much lower-hanging fruit to exploit in order to detain you. This method seems to require too much trouble and too much competence to be useful for them.
