To the OP
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I would wait a while before digging up the rocks to find the fish. If it's still alive, a sqirt of food with a turkey baster behind the rocks may let you catch a glimpse of it.
If it's dead, chances are you'll never notice a spike in ammonia or nitrites unless the system is virtually brand new, and most clean up crews will man quick work of the carcass so you may not ever find evidence.
It seems firefish can just disappear forever into hiding at the slightest aggression from a member of the same or different species, and presumably die of stress & starvation.
My Observations
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Steve is right about the low chances of getting a mated pair at a LFS. When we see a pair that "looks happy" together at the LFS they typically are in a new environment with other more aggressive fish and just haven't gotten around to targeting one another yet because they have other things to worry about. (My opinion)
However I am a medium term exception and have a ~1.5-2 year pair of Purple FF in a 65g reef, lightly stocked with "nano" fish. They appeared to be a true pair (for over a year) as one is slightly larger than the other and they sometimes hover close together gently interacting and even saw them "kiss" on one occasion. They are confident fish at the top of the tank's pecking order and even dart at other fish like a Yellow Assessor. They were out frequently and were not particularly skittish like many FF. They would often stay out in the open while I used a mag cleaner or if I had my hand in the other end of the tank. Very atypical for FF. A pleasant surprise.
But this happy event may have run its course. I added a Royal Gramma about 6 months ago and it seems the tank dynamics are changing. He's grown and is now the dominant fish in the tank. He takes prime position at feedimg time. I've never seen him be overtly aggressive towards anything but did see him flare his gills at the FF a few times during feeding competition. Now the smaller FF stays mostly in the rocks and the larger one is no longer king of the tank, has lost his bold personality and darts out for food and zips back quickly behind the rocks. I have not seen them move as a pair in a few months now. I just have a suspicion that the pairing is broken and I am going to have one FF soon. IME FF don't necessarily "kill" each other. It's more like one simply stresses out, hides and probably starves. So yea, true bonded pairs are hard to maintain, but probably possible. I think my situation would have remained stable had I not decided to add another fish.
