I had EXACTLY the same thing happen to me. Both lights definitely on when I left work after night shift at 5am. On the way home noticed one light was out seeing the reflection from the car in front. Got home and switched the lights off and on again, got out of the car to take a look and the affected light was barely on. The next morning after night shift I was looking at the lights reflection from the cars in front most of the way home, both seemed fine. Got home and had a look, the light that was giving trouble the morning before was absolutely fine and looked exactly the same as the other side, and has been fine ever since. Strange isn't it?