I can't tell you about the A3 but on the A4 (B5 chassis at least) there
are pad wear indicators on the fronts. These will cause the instrument
light warning you describe, when one of them wears down to the limit.

There are no corresponding wear indicators on the A4 (B5) rears; visual
check is required every 3-4 months or so.

You say you "were told" that there were no wear indicators on the front.
 Two possibilities:

1. The person telling you this is wrong. Remove a front wheel and look
for some wiring and a black plastic D-shaped connector, close to the top
of the wheel knuckle. You should find it mounted on a bracket.

2. Provision for pads with wear indicator wiring exists but in this
particular case some after-market pads without this were used, the
person telling you knows of this, and the wiring on the car was modified
accordingly.

Find out which of these is true. In any case, I bet that a visual
inspection of the pads on all 4 wheels will show that you need to
replace something.

Let us know what you find.

/Robert

My 99 A3 has pad sensors. Suggest you take the wheels off and check the
pads and sensor leads. Mine only has a sensor on the nearside (left- UK)
front wheel.