Peugeot 307 do not start - Emission Control System Light On

Hello there

I have a Peugeot 307 hatchback 1.6 S, petrol, manual.

It's the third time that when I return to the car it would not start at all. All the electrics are working but the engine does not turn over at all, no sound. Also, the emission control system warning light stays on. I have called RAC twice already, in first time the engineer couldn't find any fault but mentioned that could be any electric fault, so he advised to take to one of the RAC approved garages, he towed the car and it started with no problem, I have driven the car to the garage with no problems. The day after I went to pick it up and the mechanic told me that they could not find any fault, they had disconnected all the cables, cleaned and put it back, said that they had started the car for 10/15 times and always started without fault.
For the second time when I called the RAC engineer, he found a disconnected cable and said that it was the problem, after he connected the cable the engine started straight away. (not sure which cable it is as I wasn't there, my wife was but she doesn't know either)
Any ideas what could be wrong.

Any help is appreciated.

Kind regards

Fernando