To find a leak underneath steel pipes, check the entire waterline to pinpoint the exact location of the leak. It is important not to neglect the crawlspace during the search.
Begin by shutting off the plumbing fixtures, says SFGate. Look at your water meter to see if the dials are still moving, which indicates a leak in the supply line. If the dials are still moving, leave the water off for an hour and see if the numbers change. If the dials still has not moved, the leak is in one of the home's drain pipes. Using a flashlight, inspect the line of the drain pipes to spot the leak.