As usual :), Klaus is correct. There is a magnetic pickup ring at each of the wheels. The wheel sensors read the pulses from the magnets on the rings and sends that signal back to the ABS controller.

Since they are magnets, and they are just inside of the brake rotors, they tend to collect all of the steel that gets worn off of the rotor.

The way I cleaned mine was to use brake cleaner and an old tooth brush.

On the rears you need to remove the rotor to get to them which often means backing the parking brake adjuster off.

I did a writeup here on replacing the rear parking brake shoes that might help you with getting to the sensor ring on the rears.

http://www.matthewsvolvosite.com/chat/phpBB2/viewtopic.php?p=26225#26225

The 8th picture shows the sensor ring.

...Lee