Didn't one of the Jones dogs have Fanconi that was spilling sugar but never had symptoms due to the diet being fed? Seems like I read that years ago in the Basenji mag or the BCOA. (But please don't get too excited based on my questionable memory.)
They attributed it to the diet…but nobody really knows. This was before the test, but there was some talk that some "carriers" may spill a little sugar, but never have significant metabolic changes.
There have been some lines of Bs that have NEVER had any dogs spill sugar, that are having some dogs test as carriers and affecteds. The pattern of inheritance with these lines is true, so it doesn't seem to be a problem with the test, but more of something that these dogs have some sort of modifier (Lisa can probably word this better) that supresses the expression of the Fanconi.