ABS light on mercedes e500 4matic

A wheel speed sensor problem - would that come on when I am driving??? I have seen the ABS/ESP light come on when I am in idling. Its intermittent. For instance it came on last morning. In the afternoon I started the car - the light went away - I drove over 100 miles at 80 - 100 MPH and it did not come on and I returned home and the light never came on.