Is there anything else alive in the tank? He probably died and was eaten. It's amazing how fast it can happen. Look for telltale extra debris...little white bits and perhaps a scale or two, some little strings of white, gelatinous stuff.
It is probably an asian upsidedown cat. They are nocturnal and will wedge themselves under an ornament or something during the day. You will probably see it again tonight after the lights are out.
If you have a lid, he didn't get out through the cutouts in back for the filter, did he?
Some fish will jump through the smallest hole. When I was a kid I had a little 1.5 gallon globe with a tiny hole at the top for feeding (yeah, it was a cruddy tank but I was 10.) I put the fish in the day before and the morning after found it on the floor, he had jumped out through a hole the size of a quarter.