This is probably a really dumb question given the lengths you have gone to to investigate the leak--but do you have an air conditioner?
I had unexplained bilge water in my Bene 31, which turned out to be overflow condensation from the AC drip pan. They had installed a "safety" hose that drained to the bilge from the drip pan at the same level as the main hose that pumped condensation overboard. Since that served no purpose that either I or my dealer could imagine (an overflowing drip pan would also go into the bilge), we plugged it. Dry bilge ever since.
s/v Dancing Days
Lewisville Lake, TX