Yes, the members of the spiney eel family are fairly noptorious for hiding. They enjoy burrying themselves in the substrate. often you will see the tip of their nose poking out from under your substrate. It is generally best to have a sand substrate of rounded gravel substrate without sharp edges when keeping these fish. If you do have an undergravel filter, this could be a problem. Try turning off the lights and waiting about 20 minutes. If he doesn't come out on his own, try adding some food to temp him to come out. Frozen or live bloodworms or blackworms are great, and you eel will love them and should find them hard to resist. They are primarily nocturnal, that's why you should wait a while before feeding them or looking for them. Good luck!!
\Dan