well, i'm too new to all of this to have a good answer, but if it were me, and i were in that situation, i would probably check through the sand and stuff that i can get to without a major re-arrange, hopefully get lucky and find it that way...
and from what i've seen thus far in my 2 weeks of having a saltwater tank, a water change cant really hurt anything, so i'd say go for that either way, cant hurt, plus in doing that, you might just float him out of his hideing spot...
sorry to hear about your loss.