Chances are they are dead if you dun see them for a while.
Depending on what fishes you are keeping, some fishes likes to hide by nature but not the fishes (tangs) u mentioned. If they hide, the signs are that they are stressed becos they are bullied by other fishes. When they hide, the signs are not good and they will generally starve to death.
Fish dies for many reasons, some fishes have high mortality rate i.e they do not survive well in capitivity (like cleaner wrasse, most butterfly fishes etc), if hardy fishes to die, a few reasons are that they are stressed becos they have not been properly acclimatise, if they are caught using cynide, they will die over a few weeks. So do some research on fishes before buying to see if they are the hardy type (easier to keep), some fishes find it hard to survive in new aquariums, preferring matured ones (>9thhs) with some algae for them to peck on.
Most fishes can tolerate imperfect water parameters unless it is way off. Do you now that some fishes release ttoxis when they die? If u dun, pls check out.