Definitely look inside the filter, and also inside all decorations.
I have had many cats, and many fish, for many years, and only had one
incident of a cat swiping a fish out of a tank. It was actually a bowl and he could reach in from everywhere on the top and see all around the top and went a bit nuts.
Even so, that was when he was a kitten. he's 7 now and that was it.
It really is almost never the cat. Keep looking, I've found some fish in weird places. I once had a betta jump out and flop an amazing distance. When I found him he was dry and I sorrowfully went to flush him when his whole body convulsed and I realized he was still alive! I got him in water and gave him lots of TLC. All his fins sloughed off and he was a sorry sight, but he lived for several more months after that.