I have a frogfish, not the typical genus you find in the aquarium trade, though every once in awhile they do come in. Anyways, I got it back in December '08, I believe, and kept it until the end of January '09. At that time I went to Florida for vacation and he escaped from his 'pen' while I was away. Despite repeated attempts to find him with the flashlight (even getting up in the middle of the night and going in to work to look for him), I never found it. I figured my anemone or my big scoly had eaten his remains after he'd died. Both the scoly and anemone will take out pretty good-sized fishes and digest them very quickly (after they've died and floated into their tentacles). This was an important species for me, so I was fairly upset that it was gone.
In all this time, I've only had one unexplained disappearance of a fish, and it was a newly introduced 6-line wrasse. Today I was searching for a coral frag that had gone missing, and I saw, unmistakeably, the outline of this frogfish. He was pressed up between a rock in the back and backside of the aquarium, so I grabbed a pair of tongs to try to catch him. As I figured though, he wriggled away quickly. But now I know where he is, and I will be tearing out some rock tonight.
The biggest question I have, though, is how in the world did he survive all this time!? ONE fish gone, in three months. There are no hermits in my tank, but there were three emerald crabs. They're hard to find though, but I do see the big one on occasion. So assuming he ate the other 2 and the wrasse, could he really have survived that long? I'm flabbergasted.