ABS and Check Brakes lights randomly coming on

Hi, my question concerns my 1999 GMC Sierra 1500 pickup. The pickup is a short-wide, regular cab with a 5.3 liter V8 engine and 80,000 miles. For the past several months, at completely random times the truck's ABS and Check Brakes lights randomly come on. When the truck is turned off and then on again, the lights turn off and do not come on again until some other random time. The truck has never had problems with the brakes, and in fact when this problem started the truck had just passed inspection. I have noticed no problems with the brakes ever, not before the lights began randomly coming on and not during this time. I have taken the truck to be looked at twice for this problem. The first time I took it in, the mechanic inspected the brakes but found no problem. He topped off the brake fluid, hoping that that might do the trick. But the ABS and Check Brake lights came on a couple days later. After that, I took it back to the same garage where another fellow drove the truck for a bit and inspected the brakes, but he too found no problem. He opined that the brake fluid was a bit dirty, but not dirty enough to cause the ABS and Check Brakes lights to come on (but could this perhaps be the problem? Ought I to flush out the old fluid and replace it). I must stress that the ABS and Check Brakes lights come on at random times. For example, more than once they have come on while simply idling. Also, my gas guage has been giving me trouble (though I doubt it is related to this problem, since the gas guage was giving me trouble long before this problem with the ABS and Check Brakes lights). I would appreciate your help very much. I hope to avoid taking it to the dealer unless I absolutely have to.