This is very interesting. You wouldn't normally hire an investigator to dig into a corporate shitshow. Firing of Sam Altman was a huge mess, but if there was a serious reason to do that (like, frauding the board), and that reason would be backed by investigation report, it'll suddenly make board actions much more justifiable. And it'll put Microsoft into a tough spot, because they hired Altman back without any considerations whatsoever...
At this point we still have no idea what the outside board director's issue might have been but the fact that even their initial internal allies, co-founder/board member Ilya, CTO/interim CEO Mira and the COO, all stopped supporting the three outside directors after engaging directly with them over the weekend is pretty damning. Unfortunately, the scope, conduct and results of any such investigation are all entirely under the control of the three outside board members. The same outside directors that abruptly fired the last interim CEO and the CEO before her in a period of 48 hours. Unlike most boards, they aren't accountable to shareholders, investors, employees or anyone else.