I have the same problem. My missing fish l;ist is smaller though.
I've lost a green cromis(hen I first established the tank) maybe an inch in size, then recently about a month ago my green clown goby dissapeared without a trace (half an inch).
My water parameters are near perfect(just have a spike in nitrates, but thats recently)
I have a 10g tank, no anem's, just small corals frags. Got a XP1 canister filter on it(intake has a mesh on it, outake is in my fuge). I also have a 18inch HOB refugium completelt filled with cheato and LR rubble (no room for fishies to be swimming in there. I also checked to make sure)
I acctually do hear a "clicking" sound at night time, since its in my bedroom, I was thinkig it might have been a hitchhicker pistol shrimp. I havent seen anything though.
My cleanup crew is pretty much accounted for, dont know about the snails though.
My clowns and linned blenny havent gotten eaten or died randomly and I've had both for a while now, so I doubt its a disease.
The cromis and the green clown goby went missing in a span of maybe 5 months.
Current livestock is:
Asorted snails (nassarius, cerith, turbo, astrea)
3 blue legg hermits
3 hollowen hermits
1 cleanner shrimp
1 serpant star
1 coco worm
1 green emerald crab
1 clown
1 linned blenny
Do you think the clicking could mean theres a mantis, or even a pistol? If there was one, wouldnt my clean up crew be gone already?