The air that enters the cabin is indeed ducted from the engines or from the APU (Auxiliary Power Unit). It comes from the High Pressure Compressor.(It's of course air that has not participated in the combustion process.) So inside the cabin the smell will be the same as close to the engine inlets. In flight this will normally be fresh, odourless air. On the apron however the outside apron-smell might of course enter the cabin! So on the ground a "strange" or kerosine-like smell is usually nothing to worry about. If the smell continues a while after take-off, then this is a matter that's worth looking into.
But very often passengers will mistake the odour from the bread in the oven or the perfume of an other passenger for a "strange" smell.
As already mentioned, carbon monoxide is odourless !