A4 ABS and Brake warning lights

[alan page]

Hello,

A friends '98 A4 1.8T displayed both the ABS and Brake warning 
lights (with the audible warning) recently and she had her 
mechanic look at it immediately. He checked the entire braking 
system and couldn't find anything wrong, and the lights were off by 
the time he looked at it. Since then, it has happened several more 
times, and each time she has no trouble stopping the car.

She's going to have either another mechanic or the dealer look at 
it, but I can't figure out what type of failure would cause all three of 
these indicators to go off at the same time. If an ABS sensor were 
bad or one of the sensor wires were intermittent, wouldn't the ABS 
light be the only one on?

Thanks,
Alan
Newtown, CT USA
'90 80q
'94 S4