A pinhole plumbing leak in a supply line can leak plenty of water. I had a customer call and discover that when he was hanging a picture, a nail penetrated through the wall and into the pipe. Things were okay until he pulled out the nail.
For small repairs like this in copper, you can use a repair coupling. There are two types of repair couplings; one with internal stops and one without. You want to use the coupling without stops. This will allow you to move the coupling (also known as a sleeve) freely up and down the length of the pipe. To get the sleeve onto the pipe, you will need to cut the pipe at the pinhole location, and the coupling will span the damaged area. You will need to clean and flux the inside of the coupling and the outside of the pipe. Then you can slide the coupling into place.
Light the torch and heat up the area. Touch the solder to the opposite side of where you are holding the flame to make sure the entire joint is hot enough for the solder to flow. Once the solder melts and flows into the joint, you can turn the torch off, clean up the area and turn the water back on to check for leaks.