ABS and ASR lights on..Any ideas???

[95s420]

This one has got the MB techs stumped. Three times now while driving both the ABS and ASR lights have come on at the same time. They remain on until the key is cycled. The weird thing is that the techs swear that there are no codes in any of the computers. Any ideas?? I was told to check the cruise control next time the lights come on so I will report back with the results ASAP. My "amateur" opinion is that it is either a brake light switch or more likely a speed sensor. The wiring harness has already been replaced a while back so that can be ruled out.

1995 S420 90K miles