A secret cemetery?

When you walk down the path leading from Grange Road entrance to Astley Ainslie hospital grounds towards SMART prosthetic centre, there are a few plaques on the grass. I'm guessing these are graves of pets, or possibly dogs that used to guard these grounds? The oldest one is this one from 1928 and the most recent one is a brass plaque for Nipper from 2000.

Uncle Google has nothing to say about animal graves at Astley Ainslie, which barely ever happens? Intriguing...