Edit: on the GPL front, GPL doesn’t require that you publish your code to everyone, just to the recipients of your binaries. And you only have to give it upon request. So they definitely could keep it somewhat under wraps if they wanted to.

When I said "hidden from the public", I was meaning refusing to disclose the source code even when asked. I do wonder how the laws would apply to government organizations violating copyright 🤔. Like what if it was the OS for some defense system? I'm not sure a government would be too keen on disclosing that — even if it was requested though some sort of freedom of information request (if the respective country has that) — and would rather classify it and refuse to disclose regardless of the license. I'm not aware of any precedent of this.