Fault Lights Showing But No Faults

Vehicle = 1.9D Xsara Estate. Any geniuses out there who can solve my problem? Prior to an 80K mile service I was having problems starting the engine. Phoned the garage to book in for the service and they diagnosed a problem
with the glow plugs which would be changed during the service. In the meantime, to get over the starting issue, I was advised to switch on ignition wait for glow plug light to go out then switch the ignition off - and to do this 4 -5 times and it seem to help. However, the constant switching on and off caused two warning lights to show on the dash that wouldn't go off after the engine started. Garage said they would reset this during the service but they have not been able to resolve the issue as they say there is not actual fault logged in the ECU!!! The fault lights are Orange one for the ABS and a Red one for the brake fluid. The ABS works fine and there is plenty of brake fluid in the reservoir. Now they are saying they want the car for two days to trace a possible wiring fault??? Any suggestions?

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