Do you have alot of plants in the tank which could hide it? Do you have ornaments which it can hide inside?
My tank is like that. Every now and then I'll notice a fish missing..like a neon tetra. I would search and search and cant find it. Even after doing a water change..no sign of it...no bones no nothing! and my tank is only 6 gallons!
I figure between my fish, my amano shrimps and ADF...nothing gets left on the gravel bottom for long...