not a pup and peeing on my couch

[jeepers43229]

we have a small mixed dog (shelby, weighs about six pounds) she is about 4 years old. she is crated when we are not home and knows not to pee in the house. we walk her pretty frequently. i also have an eight year old boston terrier he seems that he would die before peeing in the house and he lets us know when he needs to go out. a couple days a week shelby will pee indoors and it is alway on the couch or sometimes on our bed. uuugh y? she has seen the vet and there is no infections causing her to not hold it. i don't know what to do i can't stand to crate her when we are home but she will jump up there and go so quickly that you don't notice until you sit in it or you will see her sneak away when you walk in the room she knows she did bad!!! I have had dogs in the past that have had accidents in the house and they have always peed on the floor or near the door. any insight would be helpful thanks L and Shelby P.s. sorry for the long thread