Clowns can get into the tiniest holes and cracks , especially when they're young. When my guys were new I remember not seeing them for ages and then when I figured that I'd lost them, there they'd be. Now that they're bigger I see them most of the time. Don't panic yet. He may turn up. Clowns don't tend to be jumpers, even though they can whizz up and down the sides of the tank at a real speed. Did he look nice and chunky or was he really thin? And how long had he been in the tank?