First make sure they are not hiding or in the filter or refugium, or on the floor, behind the tank, etc.
Second, what kind of shrimp do you have and is any live rock in the tank fairly new? A few shrimp are predaceous and live rock sometimes hides mantis shrimp and fireworms.
Third, what kind of hermits do you have? Several are night time predators that can kill and eat fish up to twice their own size, these include Red hairy or white spotted, starry eyed, white speckled, redstripe, polkadot, orange claw, and red reef hermits.
Last, Banggai cardinals are reported to sometimes become aggresively territorial against other cardinals, damsels, and even clowns, if they severely injure or kill a fish the scavangers can quickly make it disappear.