Firefish are extremely shy and timid fish. They need plenty of nooks and crannies to hide in, and even then it's a toss up if they will come out... Mine used to never come out as the tank sat right near a walk way and any time you walked by he was gone, and it got the point where he never came out except to grab a bite to eat and pop back in his hole... now he is used to the traffic but he still hovers right above his hole and never ventures more than a few inches from it...
I had a kliens who vanished one day in about an hour, no idea where he went still MIA to this day, though I believe he jumped out to become carpet jerky and my dog at him.
I alos had a couple peppermint shrimp that i assumed were dead as I had a sixline wrasse who shredded a few of their brethren, but upon tearing down the tank I found 2 of the 5 still alive hidding in a rock... that was about 6 months in total that I thought they were dead and never ever saw them.