I have a 2011 LX570 and I've noticed that sometimes when I put it in reverse, the most extreme back up warning sounds immediately, i.e. it is indicating that I am too close to an object and am about to hit it, and it won't go off, DESPITE there being nothing behind me and the sensors being clean. In the same parking situation, sometimes it does it and sometimes it does not. Is this normal? If not what is the issue and how do I resolve it?