In the wild, grey squirrels hoard their nuts for the winter, building up a supply of food and burying each nut in different areas so they have food through the colder months. When squirrels are taken in as pets, their natural tendencies to hoard do not go away. Instead, squirrels adapt their hoarding and burying techniques to survive. Even if it means burying acorns in some unusual places!
Of course, you can always give your pet squirrel some squirrel food blends, rather than nuts. Your long-hair dog might appreciate that!