Hissing noises are one of the most commonly reported symptoms of a slab leak. Copper is an excellent sound conductor, so the hissing sound a leak makes will travel many feet along the pipe and even become amplified at places like shower valves and fixture shut-off valves (angle stops). If you think you hear this sound, there are a few steps you can take to narrow down the source and eliminate other potential sources:
Rule out the faucets and the toilets. Toilet fill valves commonly make a similar-sounding hiss when they start to leak internally.
Rule out any fans, air conditioners, etc… Bathroom fans and AC units are also known for making noise that can sound eerily like a slab leak in the right circumstances. If any appliances are making noise while you are trying to diagnose the issue, it will be much harder to pinpoint the true source of the sound.
Next, listen carefully and systematically under all the sinks, behind all the toilets, and at all of the shower valves in the general area of the sound. Get your ear as close to the fixture shut-off valves as you can. Listen for any spot on the plumbing system where the sound is noticeably louder than everywhere else. If you can confirm that this is the case, there is a strong likelihood of a slab leak.
If you cannot hear anything conclusive with the unaided ear, you can also use an automotive stethoscope placed against a shut-off valve to verify the sound. You can even use a long screwdriver with the tip pressed against the pipe and the handle pressed against your ear.