Nothing to do with ancestry, it's what's known as somatic (of the body) mutation. The e allele which causes yellow pigment is particularly unstable and often mutates back to E (black in most breeds) while pigment is being formed during embryonic development.
You can find odd dark hairs on most dogs that are yellow due to ee - Labs, Goldens, red Cockers, Westies, white/apricot Poodles .....
This website shows an extreme form of it but gives a slightly different explanation to the one usually accepted -
http://www.labbies.com/mosaic.htmJulie