Warning light for exhaust

Well this week has been a mystery with my Octy. Had service three weeks ago so thought everthing was ok until when going to start up it did not start which was a first. Wated for thr glopwlugs to warm up and it went ok. The warning light went out after a while but came back later.So went down to my Skoda dealer and they put on the "tester" and it seemed that no confirmed items came up . They said it could be anything from glow plug , loose wire and so on . Whilst the motor is working fine just keep aware of it otherwise it might be a costly investigation or next to nothing. It has done 117 500 miles , regular servicing  since new and pulling my caravan a good mpg 27/34.