similar to "out of sight, out of mind", but slightly more insulting. Refers to someone who is your best friend/romance when with you, but seems to forget your existence when not in your company. Came from a trip I took to the beach with my dad and our family dog. I asked if the dog remembered going to the beach when he was a puppy. My dad said, "He probably remembers being here before when he's here, but when we go home, he doesn't remember coming to the beach."
Bill is really sweet and attentive when I see him, but he never calls or emails me. He's a total dog at the beach.