Some chestnuts have the occasional white hair scattered through their coats and may be carriers of the sabino gene. The sabino gene's expression varies from horse to horse. Some have extreme roaning/white markings. This horse, Airdrie Apache, shows one extreme: http://www.painteddesert.net/apache.html.
This page shows a range of the sabino patterns: http://www.whitehorseproductions.com/tbcolor3.html Note that the stallion Marquetry is apparently a chestnut with a lot of white markings; he has some white hairs scattered throughout his body, but not a lot.
It would help to know what bloodlines your horse has. If your horse is of Thoroughbred breeding, especially if she is inbred to Northern Dancer, she may have the sabino gene. FWIW, there are several breeders of Thoroughbred horses that are concentrating on producing horses with loud color patterns, so it's becoming more common to see Thoroughbreds that would be considered to be paints or pintos. Some Thoroughbreds with enough white are double-registered as Paint and Thoroughbred.
To date, Tri Chrome is probably the most colorful Thoroughbred that had a reasonable racing record, but because several very popular Thoroughbred bloodlines have the Sabino gene, I feel it's only a matter of time before we get a really, really good Thoroughbred stakes winner with a loud color pattern. In Australia, there's a top racehorse that is almost certainly a carrier of the Sabino gene, Apache Cat: http://www.theage.com.au/news/horse-racing/cat-goes-after-cream-of-spring-races/2005/09/17/1126750167073.html Bad luck is that he's a gelding, so he can't pass on his color pattern.
In general, horses that carry the Sabino gene have high white markings on their legs, generally on all four or three out of four; they have blazes of white on their faces, and sometimes extending all down their muzzle and underneath their jaw, and they may have patches of white on their underside or high on their legs.
Sabino is NOT the way you describe any horse with white markings. It's basically a gene that can be carried by horses with extensive white markings, but the only way you can find out for sure whether your horse has the gene is to have one of the many labs that do genetic testing on horses tell you. This gives some information on the testing availability: http://pacificpintos.com/articles/genetic_testing_color.html
In any case, your horse's color isn't going to change significantly, although the flaxen/white hairs in her mane and possibly in her tail may become more evident as time goes by and many horses show some seasonality about the brightness/depth of color of their coats. And of course horses are subject to sunbleaching if they're out all day, and over time this will make your horse's mane and tail get lighter.
With regard to the Birdcatcher spots, these are not isolated hairs scattered throughout the coat. They are discreet spots of white that may suddenly appear in a solid-colored horse in middle age. They're called "Birdcatcher" spots because the horse on which they are best known was the Thoroughbred stallion Birdcatcher, and many of his descendants showed these spots. However, they are not limited to Thoroughbred horses. From your description of your horse and her age, I would say Birdcatcher spots are not what you're seeing.