I'm curious what you think "evidence that it never happened" would look like, except for looking like the absence of evidence?

What would evidence that there was never a global flood look like? One example might be "lack of a global species disruption in the fossil layer". But that's just absence of evidence again.

You're talking about the idea of the Genesis flood, occurring some 4500 years ago, drastically rearranging the recently-created Earth's geology, wiping out every living thing except for the few that survived on a boat that landed on a single mountain in what is modern-day Turkey?

I think there are some predictions you could make there. However every single one of them is just so far divorced from what we observe that they aren't even worth enumerating.

Absence of evidence is not "proof" of the contrary, but it is evidence of the contrary.

"Proof" is a complicated word. People typically take it to mean "slam-dunk, 100%, without-a-doubt proves the case." But little in science ever works like that. Math, sure, where you can prove things analytically. But when trying to "prove" things about the universe, you have to collect bodies of evidence, and see how well that evidence matches expectations.

So, when you go out looking for something, and despite your best efforts, cannot find it, then that may not "prove" that something did not happen, but it is good evidence in support that it did not.

That depends entirely on how hard you looked.

Despite centuries of scientists looking hard for evidence of it (and knowing what kind of evidence they are looking for, adn being able to get it), they've been unable to get any. That's as strong evidence of the inexistence of a phenomenun as you can get.