With chem and bio, all bets are off.
“All bets are off” is unclear. If you are worried about what someone might do to you in such a scenario, then it is not entirely a SHTF thing. In a more ordinary situation, you would need that wonderful White House phrase, “plausible deniability”.
For instance, if someone finds or steals food or liquor, and eats it then gets sick, it really voids any warranty over it, especially if there is a possibility that it could have become tainted naturally.