Seem to recall a few years back an interesting Volvo abs fault.
almost undiagnosable,
turned out to be tyre wear,
the rears were more worn than the fronts,
and the different tyre diameters were resulting in different rotational speeds which confused the abs computer to hell and flagged the abs light with no stored codes.
To prove/disprove would need live data scanned from the ECU with the vehicle in motion to see if all 4 wheel sensor outputs were the same.

1/4drive
December 2014