Not sure if the theory is it is coming from a drain or where that came from, but certainly it is important to check the drain hoses, since a failure could sink the boat. A really good idea to keep the thru hulls closed.
I brushed against my sink drain hose while putting something under the sink, and it started oozing water…it was so rotted ! It looked on the front facing side, but the hose was totally shot.
If not coming from the drain hose, then following the water lines from the a tanks, pump, filters, etc and check every fitting. If no luck there, then you have to start looking for signs of water under cabinets and places you can’t easily follow the lines.
Dont forget the water heater, cockpit shower, etc.
I am a bit perplexed by the idea that it only leaks if the tanks are “partially“ filled….that doesn’t make sense to me.
I could isolate my port and STB tanks….If I wasn’t sure where the leak was, I would probably start with one tank isolated and see if I get the leak. If not, switch to the other tank and see. If it leaks regardless of what tank is used, then it has to be leaking somewhere after the place where the lines come together.
Greg