I had this problem for about a year and a half. Has I would be driving I would notice that I had a rear light failure warning indicator. This is item number 12 on page xii of the pictorial index in my owner's manual or the little yellow indicator at the bottom of my instrument cluster display far right. Has time went on it happened more often. The indicator would come on and I would look for a rear light to be out but found nothing. Finally, every time I used my SUV the light was on. I began to believe I had a bad sensor until one day I noticed that when I first started the vehicle the light was out. The minute I put my foot on the brake to place the machine in drive the light came on. I went back and looked at my rear lights again and noticed that the center top brake light was out. You must have someone to step on the brake wile you look to see this or use the reflection in a large store glass window. When I finally prided off the inside upper hatch trim to gain access the the lamp assembly I found that the light was burnt out. Not only that but the bulb was loose in it's socket. If you held the socket so the bulb was facing down it would fall out. I took a small screwdriver and pried the two ends of the contact points closer together so the bulb would fit snugly. I put it all back together and its been working like it should. Vic

 

Had a similar problem with my RX300. Sometimes the light would work, sometimes not. Turned out to be a broken filament that would make contact under the right condition.
Changed the bulb and no more problems.
Took a while to find the problem.

 

I realized after 2 replacements that the brake bulbs might burn gradually with time and changed their resistance enough to trigger the warning (can not be seen).

Make sure to change both sides (~8 bucks a pair) to avoid my mistake that I forgot which side has been changed during the first warning :-)