I don’t think Ariel associates the door with letting us know it’s her bathroom time. Instead she’ll come over to us and quickly/firmly “nibble” our hands or beside our knees. If we ignore her for a short time, the paw comes up to swat our knees. It’s the absolute only time she acts like this so it’s a no-brainer for us. That being said, I don’t know how she’d signal someone other than us who wouldn’t understand what she’s doing!