Our yellow hid for 9 days when we put him in the tank. We discovered he was coming out when the lights were off & we were at work. He eventually got used to us. Then one day he disappeared. For 30 days. We figured he was dead under the sand somewhere, then one day we came home from work & there he was, waiting to be fed.
DO NOT DIG for your fish. You have to have patience in this hobby. If he's dead, you'll know if you get a parameter spike. If there's no parameter spike, then he's coming out when you're not there. If you dig him up, he'll always hide.
You can't control livestock or coral behavior, you get what you get as far as that goes. For the most part, nothing will do exactly what you want it to do. Anemones move where you don't want them, corals grow the opposite direction, shrimp, snails, or fish don't "fix" what you bought them to fix or act how you want them to act. This is nature & cannot be controlled by you. What interests me in my tank is watching nature take place, and if something does exactly as I want it to, then I'm very happy LOL. Otherwise I just watch & learn & enjoy.