Why? I don't think UFAI can kill us in mere hours/minutes, and at some point during the months or years it will probably take, someone, somewhere, will notice what's going on.

By the standard of "someone somewhere noticing" we have proof that aliens are on earth. Nobody will take that as a way of resolving a bet.

A powerful UFAI can control cyberspace before it kills everyone. You would need cyberspace to function to resolve a bet.

It took a while after WWII till Italy's public recognized that the Mafia was back. An UFAI would make a mistake at waging infowar for it's identity to become public knowledge. If you believe that UFAI's would go FOOM then they are unlikely to make mistakes.

And for my apparently abundant downvoters, the whole reason I'm asking to take bets is because some people repeatedly profess to believe precisely that every fresh machine-learning advancement is a dangerous step closer to a very sudden self-improving UFAI that destroys us all.

You get the downvotes because the average person who holds that belief has no reason to take the other side of the bet. Getting money on doomsday is not a valuable outcome.