There was a suggestion in Elektor magazine a few months ago, using a small incandescent light bulb as a sensor. You have to break off the glass bulb without breaking the filament, then run a constant current through the filament. Gas flow over the bare filament will then cool it, changing its resistance. So, you can sense a varying voltage across the filament according to gas flow. This is how "mass flow sensors" in car engine management systems work.
Might be a bit invasive for you, though?