NOTE: This is not the same as the rhetorical question used to incite someone’s curiosity, in which we usually omit DO and sometimes even YOU. “Do you know something?” asks if there’s some information you have that you’re perhaps withholding.

что-нибудь через дефис

Ты (ведь) что-то знаешь?

👍🏼 It should sound suspicious

Ты что-то знаешь? Говори!)

asks if there’s some information you have that you’re PERHAPS withholding

Is this really an imprecation that the other party withholds some information?

Without any context, that would be the most likely interpretation.

implication

Is this because of "something" instead of "anything"?

Can this mean just "Тебе (об этом) что-нибудь известно"?

Exactly. It’s because they used something instead of anything. And because they used “do you“ at the beginning.

To me, Ты что-нибудь знаешь? & Тебе что-нибудь известно? don't sound suspicious. Что-то instead of что-нибудь would make them sound suspicious.
Ты что-то знаешь?
Тебе что-то известно?

Exactly! Because it’s asking if they know something specific.

🙏👍