i would seriously consider telling your mom to try taking the keys out of her pocket or wherever they might have a chance to have the trunk release button pressed, while it may seem like the keys need to be close to the car in order for them to work that is not always the case as i have noticed depending on what angle im aiming my keys at my car they either will or will not lock/unlock my car from a far distance. not saying that she is accidentally pressing them, but just trying to get you on a process of elumination to see whats causing it as this is the first i've heard of the trunk opening by itself on a versa.