My money is on the cat! I had a snowflake eel in college and my roommates cat walked into the living room with it in dangling its mouth when it decided to carpet surfing. Over my 20 years I've had a few fish disappear mysteriously without a trace. After my blue spot AND Splendid Pintail jumped through the mesh recently I now have a indoor Ring cam setup on the tank so I can go back and see what the heck happened lol