First, you can find the leak by inflating the tube (outside of the tire) and immersing it little by little in a basin of water going around the whole tube. You will see the steady stream of bubbles coming out of the hole. If the stream of leaking air is fairly strong, you can run the tube by a sensitive area of your skin (like just below your bottom lip) and feel the air escaping.

Second, pinch punctures or weak spots in a lightweight tube that just give up and open spontaneously can result in either slow or fast leaks. There is no rule. Over time you will get all kinds.