I believe the answer would be sweat
Body fluid that hard to be seen by naked eye must be clear(have no color) and not thin. Blood would be out of the question as it red color will be easy to perceive. Semen color is white but it will be clear after a while, but it was thick and has a strong smell.
Saliva is clear but mostly it has some bubles and a bit thicker than sweat. Sweat would be the hardest to find as it was clear and mostly water. But it gives less information as evidence than the other fluids.
