A faucet mixes hot and cold water inside of the faucet body. So if there is no pressure on the hot side, the pressure on the cold side pushes the water back through the hot water piping, and out of the open pipe in your garage. If you can post a picture of the open piping in your garage, we should be able to recommend a temporary cap solution for you.
Lake County, Florida plumber
Springtime Plumbing Services