Fuel isn't your only enemy...........While looking up an article on electric sun pads I found this website..........thought it might be a reminder to folks..........
http://www.carbonmonoxidekills.com/boats.htm
I know that some boats have a hole with a small cover plate mounted on the sun pad/engine hatch. These holes are for stuffing the end of a fire extinguisher into if you have an engine fire. (You never want to lift the hatch if a fire is already going. KA-BOOM!) Perhaps if you have an electric cover you have one of these smaller holes and could whiff thru them.
I agree with the other poster to sniff the blower exit line (but then you run the chance of the blower start up igniting the fumes).
CZ