I am looking to install a shutoff valve for an outdoor faucet, and to do that, I need to locate the pipe leading to that faucet. I tried to trace it by listening for the hissing sound when the water was running, but that doesn't seem too reliable. Any better ideas (other than cutting holes in my walls)?
Metal detector? Thermal imaging? There are utility locators that pick up a signal conducted through metal by a transmitter. Even with non-metallic pipe sometimes you can remove the faucet and push a wire down the pipe. Acoustic detection via electronic means is also a method used.
There are contractors the offer location services that have sophisticated equipment. Their fees are proportionate to their equipment costs... high. It's likely much cheaper to cut and patch holes.
On the popular shows (Holmes on Homes) I have seen them use thermal cameras while they are running cold or hot water. You would have to rent one for an hour or schedule a "green" company to come out to borrow theirs.
If the pipes are plastic, I don't know. Draining the pipe and making a flute sound inside it through the faucet is one untested idea. Tearing apart the wall will definitely work.
If the pipes are steel or copper, have an assistant stand outside and moderately thunk the faucet on the side with a block of wood, heavy wooden spoon, or similar repeatedly. I have never had much luck with a hammer, rock, or metal rod because it makes such highly pitched sounds they are hard to discriminate a distinct source direction—they seem to come from everywhere. Also, small pieces of wood (like a pencil) don't work so well either for the same reason.
Go inside and press an ear firmly against the wall and carefully listen. When you are close, it sounds like it is right there. When you aren't close, it is distant and vague. Start right inside where the faucet is and try listening at various spots a few inches apart by sliding in vertical or horizontal lines—perpendicular to the direction the pipe is traveling. I found I can get within two inches when there is no insulation, and four to six inches when there is good insulation.