Yeah, the brake light switch is definitely the most common cause of ESP/BAS malfunctions, but it will store a fault code. One of the lesser known culprits is the vehicle battery. I've had a handful of stubborn cars over the years that gave VERY intermittent ESP/BAS warnings. All other electrical functions were normal. In these cases the warning lights came on immediately following engine start. Switching off the engine and re-starting caused the lights to go off. I don't know if this information will help, since you say the light comes on after driving for awhile, but it's easy enough to check. Good luck