I'm sure I read somewhere the ABS rings can get dirty and that can cause faults with the front wheels
the rear ones are straight into the hub with some sort of magnetic pickup i think
good video
here by Sirobb/Cheshired5 shows where they are and what they look like. you can probably check / clean them without taking it apart but it's nice just to see it all disassembled so you know what you're dealing with
sensor locations front and rear:
