It's very common for them to wedge themselves into a crevice and stay there for the first week of being added to a new tank. Do double check all around the tank to make sure it didn't jump out. But My firefish was missing for almost 2 weeks and i assumed it died until I finally saw it poke its face out from one of the caves in my rockwork. So it's either hiding, or if it did die your clean up crew made quick work of it.