Is this a guessing game? Rain water, from the companionway slider, through the cabin liner into the cubby hole. Final answer!
You are going to have to get the hose out and chill that case of beer. I suggest you start wetting the cabin top in sections for 15 minutes at a time and observe. You can do the slider, then each fixture that you may have on cabin top and the porthole closest to the galley. Don't forget the chainplates and the railing in the deck to hull joint on the starboard side. Good luck, hope you find before you run out of beer.