'03 Malibu LS - had the brakes done (pads and rotors) about 2 1/2 months ago
and all has been fine until this week.  Occasionally the ABS warning light
and chime will go on, but only under specific conditions ==> car just
started, reverse, then shift to drive, light sometimes goes on.  Shut off,
then restart; light goes out and stays out regardless of forward, reverse,
whatever for the rest of the drive.  Only happens rarely (maybe 1 in 25
starts), but never under any conditions other than the above.

I figure a front ABS sensor, but really don't know why the above conditions
would do that.

In addition, I would like to avoid finger-pointing between the dealer (for
the sensors) and the brake shop (for the rotors)

Any suggestions?