I have had 2 of these now and, once acclimated, both spent allot of time out in the open. I have also seen that adults can be teritorial with other gobies, and even blenies. I would say a search of your tank is in order, either to verify that they are both ok or, in the worst case, to remove a dead fish that may pollute your tank.
Good luck, I hope you just have a shy one that likes to hide.