I'm a rotary man myself and don't profess to know much about workings of a 737, however, according to BBC Scotland News and Jim Ferguson, the well know Aberdeen aviation journalist...
I suspect this will have been caused by oil from either the Auxiliary Power Unit or the engine getting into the air conditioning system.
Once the oil heats up it creates smoke and fumes, which can be toxic. Of course, it shouldn't happen, but it does occasionally, perhaps two or three times a year in the UK.
So, there you have it, sorted...
