2014 A6 Estate - Amber Warning Light (Catalytic Converter)

[LabOwner]

Afternoon all

I have a 2014 plate A6 Estate and I would appreciate any advice you may be able to give.  

Last week I had it booked in for a service at a local garage on Friday.  36 hrs prior to the service the catalytic converter light came on, and so I dropped it off early with the garage and asked them to investigate the light after they'd done the service.  I got a phone call from said garage, at around 1300hrs on Friday telling me that they'd done the service and that their diagnostic kit had come up with 3 separate fault codes relating to the warning light - namely a glow plug code, a Diesel Particulate Filter code and a cam shaft alignment code (the cam shaft was replaced last Autumn).  The garage said that they could do no further diagnostics as their kit wasn't in depth enough, but they put me in touch with a garage with better diagnostic kit.  

Car has been with the second garage all morning today, and they tell me that they can't find any faults (possibly as the first garage may have deleted the fault codes).  No warning lights, no fault codes, zip.  

Has anyone had circumstances like this before?

Thanks in advance for any help you can give.  

LO

PS - Picture of the warning lamp in question.