I have had a similar thing happen to me too.  Me
and my fiancee were looking through our cels to
find a few to sell and he wanted to sell one of
his Vegeta cels.  He said he thought we had a
sketch to it too.  We looked a little while and I
looked on our computer for images and saw that
indeed there was one but I couldn't find it.  

I'm really absent minded too so I thought I had
accidentally put it in with another cel or in the
same page of my cel book as another cel (because I
have done this before).  I was getting a little
worried when I couldn't find it and then he said
never mind here it is, he had taken the cel out of
the book and I guess as he had it had fallen on
the floor where he didn't see it right away.

*phew*