Only thing you can do is take the plants and ornaments out of the tank for a good look, have you checked the filter, also check around the floor area where the tank is just to make sure he hasn't jumped out.
Do you have any cats or dogs who could of eaten him if he jumped out.
yeah I have 2 cats so that is possible but TBH i never ever see him come to the top but it is possible.
Another bit of advice if possible I want to add some more gravel quite a bit of it only put in a fin layer when i started I want to put some smaller over the top of the big stuff. if i empty the tank to do it, should i keep 100% of the tank water? also should i do it now or wait untill the water is better?