Little Bites
My Petey, almost 9 years old has been doing that since he was a puppy. We call it "little bites". He always did that to my arm and if another dog, any dog would tolerate it he would do "little bites" on their necks. Never hurt us, we just thought it was so cute. Now that he is older does it only once in a great while. It must be a Boxer thing!!!