Do you here any clicking noises at night? Have you seen any shrimp in the tank that look like praying mantis? With what you describe, my first assumption is either a mantis shrimp, or a very nasty hitchiker crab.
Just to cover all bases...What is your ammonia, nitrite, nitrate, PH
, temp, CA
? It is not uncommon for fish to die from one thing and the carcass to be xconsumed by your clean up critters (that's what they are there for, it is also not uncommon to never find any trace of the dead fish.