Correlation of non-coding DNA with coding DNA
If you have 2 different mutations (irrespectively of where they sit in regards to the final gene-product), and both correlate perfectly with a given phenotype, (and you have dismissed any other potential cause), you can not formally say, if the first mutation, or the second, or both mutations are required for your phenotype. (Though you can make educated guesses or rely on external information...) 

Loss-of-function mutations do not have to be in the protein-coding region (but simply in one part of the genome, which is required for the expression of a gene towards a phenotype).

Polymorphisms can also be neutral - especially in artificially bred / selected animals, such as the wolf-like animals we now call labradors. You could easily envision a scenario where some neutral mutations was present in an individual, which would have a desirable / selectable mutation. If that neutral mutation is within a few 100bp of the selected mutation, it unlikely will have been uncoupled from the selected trait / mutation.