Well, the tank definitely has a cover. There was a space of about one inch on either side of the filter, though, which I have now taped over just in case that is what happened. My cat is terrified of the tank, though, and it is on a desk the cat never goes on. If my cat did get Crumpet it was after he'd already jumped out. Either way, thank you for the advice. Still not sure if Crumpet jumped out or not but there is absolutely 100% no way another betta could jump out, so that's good. Thanks again.
Derby -- male
Cleo -- female crowntail