Hmm, that would be cool if she got the sabino marks!
She right now only has little white hairs going down her ribcage and a big "spot" of white hair on her hip. I am enjoying watching her changing colors.
Bays do not "grey out" so Id say your mare either carries the rabicano gene, or she has just always had slight roaning, that is only visible with the seasonal change of coat. I have a bay roan mare who is completly bay in winter, and in summer she is a true roan, with a brown head and legs and a grey body.
I have a little bay mare that has done the same thing. She went from solid bay with just a few white hairs here and there, to quite "roany"! Mine is Rabicano. She's developing a skunk tail and some "bars" on her ribs from the gray hairs. Her hocks and one fetlock are almost "silver" looking.