He literally told a friend shortly before that, if anything happened, it wasn’t suicide:

Speaking to ABC News 4 Jennifer, who didn’t give a surname and was described by the network as a “close family friend” of Barnett, claimed he told her not to believe any reports of his suicide some time before his death.
She claimed he insisted “I ain’t scared” before adding “but if anything happens to me it’s not suicide.”
https://www.newsweek.com/john-barnett-boeing-whistleblower-predicted-death-scandal-1879548