Check out the ignition key thing in the previous post. If nothing changes, then the next things to check are abs sensors. Most likely, though, is the controller. The problem occurs when micro-cracks develop in the pcboard circuitry - and can be corrected with solder reflow (if you access to the appropriate equipment). Cost of the controller is around $300.
--
(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)

I get the TRACS and ABS lights on at start up until I wiggle the key around and they will go off. My car needs the electrical portion of the ignition switch replaced and I will get around to doing that (with the excellent instructions on Volvospeed Bay13). It could be that you have the same problem, your wife could have hit a pothole or a bump in the road which would turn them on until she hit another bump which would turn them off. Next time you start your car wiggle the ignition key around and see if you can duplicate the fault.
This migh help.
John Scales (1996 850R)

Sounds like the ABS/TRACS control module is going south, mine also went to the ABS junkyard in the sky but I did not bother to get it changed yet.

The light comes on and off intermittently still and definitely a daily affair for my brick.