Megatrev62. I know what you mean about dismissing the theory of the predator because of the remaining small fish. I made the same assumption as I would loose a fish or two every couple of months or so for three years while a pair of Firefish were perfectly fine for the entire time period. I heard some clicking coming from my DT after making a recent coral purchase so I took a light to the tank every evening until I could find it. Turned out to be an inch long pistol shrimp that I put in my sump but I also came across a large Gorilla Crab in the process that I was able to remove and had no clue that it was there or for how long. Please heed the warnings from these fellow hobbyist of a predator in the tank or at least answer rssjsb's question about live rock and if you have any corals in the aquarium rather than simply assure that there is no chance of a hitch-hiker. Also, if you have a large CUC, they can remove a dead fish from evening to morning. Either way, if you can't find the body and truly looked in every nook... then it was eaten by something sinister or eaten by something doing its job.
Good luck on finding the cause and with the Lubbocks and any future fish.
Mike