This problem is most likely a loose connection. There won't be any codes for this. Check for loose fuses and connectors. Start at the battery and work your way towards the fuse box. It is possible for a fuse to break but not blow and cause an intermittent connection problem. I would start the car and begin wiggling things. The problem is intermittent and as you say if the light isnt on when at the shop they wont find anything wrong. I'm suprised nobody has found the problem yet. It is obvious what type of problem is occuring.