It could be the calcification process starting from some sort of old injury. It could have been an injury that didn't even lame him up; meaning you wouldn't have known something happened.
I went thru this with one of my horses and it bothered him a lot until the process stopped. I asked the vet about removing the lump (his is about 3/4" wide by 1-1/2" long) and the vet said no; that I would be asking for needless trouble, especially since this all started after the horse hit his 20's.
I agree the vet really needs to look at it but you asked for thoughts and that's mine