I really think its a battery saving feature, if your below a certain level, it will turn on, like a motion detector. I could and am probably wrong. But I tested it 3 times, waved my hand in front of it lights came on, then few secs later shut off, repeated 3 times with same results.
It's the proximity light sensor. It is located above the motorola logo on the top edge of your phone, to the right of where your ear piece speaker is. If you try putting your finger over it, it will cause the hard buttons to light up
Ok now I know its not my super powers making this happen.. it appears if your batter is below 50% or 40% and you wave your hand or walk in front of your phone this happens. I tried it twice again, and same results. So its definitely a power saving feature. I had my phone plugged in both times, I don't know if this has anything to do with it but I'm guessing not, as it would only seem logical that you would want to save your battery power more, more when its not plugged iit. I'll try and get a video of it next time... but this was 4 30 this morning and didn't think of recording anything... lol
It doesn't have anything to do with the battery level. My battery is fully charged (unplugged) and when I put my hand over the sensor, the keys light up because there is less light hitting the sensor. The key backlight only turns on when it's darker.