Years ago, I had a greyhound who was lame on a rear leg but I could find nothing wrong. I took him to the vets a couple of times but they found nothing either, even after an x-ray. Eventually, I took him to see a track vet, who said that the outside toe was dislocated at the third joint up from the ground. Once he pointed this out, I could see that this joint had a more acute angle than the same joint on the other leg, but it wasn't glaringly obvious. If you can, I'd try to find a vet that treats racing greyhounds to have a look.