Any known problems with Rear alert signal sporadically working?

Hello, this concern is speaking about the blind spot readings I believe? If so, these systems are really made for the benefit of the driver, but typically are subject to interference and inaccuracy at times. For actually guidance about the systems and help checking them out, see your local Volvo dealer -- I'm sure you have factory warranty still in effect on the vehicle perhaps -- and evaluation may be at no charge to you.