Have you removed bogwood/rocks etc.?
I wouldnt worry, as long as he aint in the filter or on the floor, hell be just hiding, just make sure hes not got out...
He's an eel right, so therefor he will have a certain amount of mobility, check the room throughly every nook and cranny. My guess he's left the tank.
BigC
IF he got out of the tank (I highly doubt) he wil only b 1-2 meters away from it.
If you haven' found him on the floor (it sonds like you have looked properly) then I VERY much think that he is still in your tank.
Give it a few days and he will come out by himself for food. Whilst you may have checked the gravel there is still a VERY high possibility that you missed him. Not to mention the fact that he might not be in the gravel anyway.
I have a tree root ornament in my tank and it is hollowed out by a hole in the bottom. A few of my eels hide in there and i don see them for days.