Been there, done that, got the T-Shirt. Over the lasT 40 years I have found mysterious leaks as follows:
Check the stuffing box on the top of the rudder post. These occassionally leak.
Check the exhaust hose, particularly where there are any sharp bends. Depending upon the brand you may find a small crack has developed that only leaks when the hose is pressurized with a slug of water from the water lift muffler, which is intermittant.
Check for cracks in hoses that attach to through-hulls, particularly at the raw-water intake to the engine, at heads, and at air-conditioner discharge through-hulls.
Check for leakage around the "water tight" bulkhead between the chain locker and forward cabin. Sometimes is isn't....
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."