They don't need to disclose how they caught it, just what said misconduct was - going "yeah we're closing your account due to misconduct" is like fining someone for no reason. It'd be a violation of human rights if that happened, and in this particular case, it's potentially ruining someone financially.

It's not really - it's more akin to closing someone's bank account and saying "we're closing your account for misconduct" which does happen.

Not disclosing the details for the reason could prevent a malicious actor to learn and improve their strategies.

Except false positives always happen, and when you have such a policy, innocent people are caught in the middle and face an accusation of vaguely defined misconduct with no details to use to try and appeal the incorrect accusation, because they don't even know what they're being accused of.