mystery of the missing fish

I was up to 5 fish in my tank. Betta Boy IV, Sucker Fish V, a white corydoras and two spotted cories of different types.
I don't know what happened with my sucker fish. One day it was just dead. So I buried it in the sacred cactus (which is so full of dead catfish that I can't dig a hole without finding a corpse) and bought a new sucker fish. The next morning I turned on the light and sucker fish did a kind of jerking spiral to the bottom of the tank, where he sat with his fin up. I said, "well, his fin's up, so he must be okay" and I fed them and left. When I got back, the sucker fish was in the same place, but the other fish were eating him. Yum. So I fished him out, noting that his fin was STILL up, went back to the petstore with a water sample and got another sucker fish. He lasted 2 days. So obviously, despite my okay water tests, my tank is inhospitable to plecos.
That left me with the 3 cories and the betta. Fine, but no one to eat the algae.
Last week, maybe Thursday, I couldn't find the smaller, darker spotted cory. This is the fish who survived the great poisoning a few months ago, when I put the dirty rock in the tank and killed my giant pleco and everything else except this one cory.
My gravel is black with some small white specks so you can see how a dark brown cory with darker spots could easily hide. I didn't think much of it. There are 3 live plants, three rocks plus the plastic house-rock where he could have been hiding. I did a gravel vaccuum thinking I'd find a corpse or he'd swim out of hiding but neither happened.
But as the weekend progressed, I had to admit that the fish was totally gone. I did another water change/gravel vaccuum yesterday, and I moved every rock. No fish. No corpse.
The tank has a clear plate covering it. The only sizeable opening is where the Whisper filter is, and there's a constant stream of water flowing out of the filter into that hole so the fish can't even get up there much less jump out.
So where is my fish??