Sable doesn't appear from nowhere so if you don't have it in the pedigree of sire or dam, it's unlikely to be sable. When you mix solids and partis, you can get strangely marked dogs so perhaps this is the case with the pup in question or sometimes what can look like sable markings disappear with age.
If you do a Google search, you can also probably find websites for breeders who have sables - perhaps you could contact one of these and ask for their opinion.
