holes in her back hooves

hi, i have an abraian mare. for about mayb 2 months she has little holes in her back feet. they r jst below her cornary band (spelt wrong). i took her the vet and they've never seen anything like that. she has 2 holes in her right hoof and 4 in her left. the vet gave me..some kind of drops to put n them but they've only helped a little. she does have white feet, which i know r softer than black ones. i have two other horses but their feet are fine.
   
i plan on contacting a farrier to see if he knows.
   
have any of you seen this before?
   
thanks

O! she isn't lame and it doesn't seem to bother her at all.