I've had a very slow leak (from a tiny piece of wire). I had to submerge the inflated tube in the bathtub, then go around it and wipe off the small attached bubbles with my fingers. At the puncture, a small new bubble would form in 15 or 20 seconds, very slow. I then wipe it off and see that another new bubble appears there. It's hard to find. This would be too slow to make a "shhhhh" hissing noise while on the bike, though.
Most punctures can be found by holding the inflated tube up to your cheek, you can feel and/or hear the air leaking. Or they spit out a stream of bubbles under water.