Don

I appreciate the scenario you describe. My on/off switch for the
sniffer is located in a sail locker accessible only form the cockpit
and is always turned on before anything else. It is located higher
than the bottom of the companionway which is is no way air tight. The
cocckpit drain exits the transom above the waterline.

There are no guarantees in life but the system as it currently stands
is good for most scenarios.

Cheers

Matt