FWIW, I posted Peter Tertzakian's mention of "missing barrels" from the book "a thousand barrels [...]" a few days ago.

http://www.theoildrum.com/story/2006/2/15/02834/6658#23

I can't find the page now, but if I recall correctly he said something like "good data is hard to come by, and experts keep their sets of fudge factors, based on their experience, from official numbers.  They apply those fudge factors to new reports."

It could just be that Simmons is on a campaign for better numbrers, and "missing barrels" is what bubble up in the press/

Simmons has been all about transparency and better data for years now. I think what he's trying to point out here is that the quality of the data could be better and that any attempt to get our hands around the problem of peak oil is hampered by the quality of the data. Ergo, get better data and we can get a better picture.