this exact same thing happened to my Eddie horse last summer. i went to test ride him and discuss leasing him with his owner one day - his right front foot looked just fine. when it came time to deliver Eddie horse to my barn he had (for lack of a better word) a dry divot right at his coronary band, just right of centre of his foot. there were no signs it was an abscess when i test rode him (and i was very detailed when looking him over, running hands down all his legs, etc), but i've had people say it is an abscess that ruptured. Eddie never took a lame step due to it, and it never seemed to bug him. it's now grown more than halfway down his hoof since the end of July last year. i think he just dinged it on something, external injury only.