Guppies can eat there own young at times, but other than that I have
never experienced guppies eating other guppies although I bet if they
were hungary enough they would eat already dead guppies. What you
describe could just take time. I wouldn't worry about it yet - give it
some time and think about it in the background and maybe something will
surface in your mind that helps you find him. These things can happen
and in the long run the creature often reappears, I am not talking
superstition it's just plain mindboggling enough when these things
happen, that's all. Recently I couldn't find one of my musk stinkpot
(sorry, off topic) turtles even after tearing the tank apart and the
next day I found them all, so don't panic - at least not yet. Good luck
and later!

P.S. - I have a lot of fancy guppies myself and what is happening to
you often happens to me at least back when I actually worried about it.
Now I just make a mental note of not finding one of my favorites and
they practically always show up later and I don't even bother to expend
much energy looking for them let alone tearing the tank apart. It just
happens. Some of my stress is also relieved now because my fancy
guppies multiply like crazy too. Good luck - and later!