There's a string stating that the code should not be passed or the employee will be fired. I'd assume this was a test to see if an employee meant to be doing code review was actually doing them. Spoiler, they were not, as OP said they found this in production code.
I'd go a step further and I suspect it's a peppering string (i.e. fixed string you add to hashes to defeat rainbow tables). I'd really hope it isn't as you mentioned because gosh that sounds like a toxic workplace if someone is just leaving landmines around purely to get someone fired.
More like, you know damn well that Jim keeps passing code reviews without reading a line in them, he's been talked to, still does it, and you need something actionable to prove it so that you can get someone's ass in his chair who does their job.
From the stories I've heard from corporate software employees, this does sound like exactly the kind of thing you gotta do to show some manager the guy is buddy-buddy with that they're actually not doing their job. And even then they didn't listen.
No, I don't think so - it's just a dick move to go out of your way to sabotage someone. If they're fucking up just visit their existing mistakes - don't waste time contriving new ones.
My guess is a "solution" to the age-old problem of needing to store a secret in a file that the user can download, thus making the entire system insecure.
This "solution" appears to be either that the string itself is so outrageous that the user would not believe that it's the real secret when it is in fact the real secret, leveraging security through obscurity, or else it's there in place of the real secret that cannot be revealed under pain of death firing, and therefore is accidentally being used instead of that intended secret... so it's not secret after all.
Unless they're doing something incredibly clever to substitute that secret string for the real thing when the time is right and doing it in such a way that the user can't intercept, someone's getting fired.
No, it's not. It's part of React internals that you shouldn't use because your app will break. It's a warning for developers using React. It's not a secret of any kind.