This is what i think. When jabu jabu first ate her when she was a little kid (my evidence ruto: i've been going inside jabu jabu since i was little) she found a bottle and some paper and a pen or something inside him. She found her way to the rectum (jabu jabu's butt hole) and tossed it. Jabu...
i think that maybe she wrote it the first time jabu jabu eated her and that he pooed it out and the water flow from zora's fountain to lake hylia carried it there. Also Ruto probably didn't want to get in trouble so she denied it. Also with complete access to the intestines you would think that...