Genetic Genealogy - verifying the family tree with shared DNA segments

This is an aspect of Genetic Genealogy that I'm sure is well known to some researchers, but that, I'm only just starting to appreciate.  I've been DNA testing for ancestry heavily for a year or two, but my prime interest has been older ancestry, admixture, and population genetics.  All of my early attempts to contact matches through 23andme and GedMatch, resulted in frustrated conversations with North American testers, that had no paper trail back before their ancestors emigrated.  Today, I matched on AncestryDNA with a third cousin.  The DNA prediction was fourth cousin, but the relationship on paper is third cousin.  This was my third match, confirmed by both shared DNA segments, and by a shared paper trail to common direct ancestors.  How cool is that?  Finding that yourself, and other researchers, share segments of the same DNA that appear to have been inherited through recent common descent.  Finding each other through the code of Life that is in our cells, and being able to see where that DNA came from in our family trees!

The image at the top of this post represents the biologically verified tree, as represented by colour shaded areas of my pedigree fan.  This is based on descent from shared ancestry found in DNA matches.  There is always the slight possibility that we share DNA from other unknown or unrecorded routes.  But the probabilities are high, that these shared segments of DNA do come from the known common ancestral roots in our trees.  The stronger the verification, perhaps through multiple matches, the darker the shade.

This discovery of a third cousin on AncestryDNA, combined with my mapping of the correlations between paper trail and DNA matches serves as an incentive to work harder on finding and contacting matches.  I've also spotted common DNA segments with someone that flags up as a fourth cousin ... but according to our shared paper trails, and family lore, should be a second cousin.  I'm trying to get a response from the tester.  But have I uncovered another family secret?