That's a sad tale, retrocop, and certainly reminds us that we shouldn't rely only on our pooing on the windows - if there is any sign of something amiss in between tests, the GP should certainly be made aware a.s.a.p.
However - from the point of view of economics, the windows must be cost-effective, colonoscopies are more costly and the numbers of people falling into the screening programme age bands continue to rise.
It's the same as any screening, IMO - there will always be cases missed, but this does at least get us in the way of having ourselves checked out on a regular basis.