They aren't secret and the fact you'd make that claim pretty much completely invalidates your opinion.
https://ustr.gov/about-us/policy-offices/press-office/fact-s...
> Full transparency in cases. Governments must make all pleadings, briefs, transcripts, decisions, and awards in ISDS cases publicly available, as well as open ISDS hearings to the public. One key objective of these provisions is to allow governments that are party to the agreement, as well as the public at large, to carefully monitor pending proceedings and more effectively make decisions about whether to intervene.
> Public participation in cases. Tribunals have the clear authority to accept amicus curiae submissions. In U.S. cases, amicus briefs have been submitted by a variety of NGOs, including the Sierra Club, Friends of the Earth, and Center for International Environmental Law. (Documents in all investor-State cases filed against the United States are available on the State Department website.)