Most of the housing across the Inland Empire and the San Gabriel Valley sits on a concrete slab with the water supply lines cast into or beneath it. When one of those lines develops a pinhole, the water has nowhere to go but into the soil under your floor, and it can do that for a long time before anything is visibly wrong.
The earliest reliable signal is the meter. Turn off every fixture in the house, including ice makers and irrigation, then watch the leak indicator on the meter. If it moves at all, water is leaving the system somewhere. That single test separates a slab leak from a running toilet faster than anything else.
The next signal is heat. A failed hot-water line under the slab warms the concrete above it, and a warm patch on a tile or vinyl floor that has no business being warm is close to a diagnosis on its own. Add the sound of running water with everything off, an unexplained jump in the bill, or a section of flooring that has started to lift or discolor, and the picture is fairly complete.
What comes next is where money gets wasted. Opening concrete to look for a leak is enormously expensive and often lands in the wrong place. The right sequence is to isolate the system by section to confirm the loss is under the slab, then use acoustic and thermal equipment to pin the location to within inches and mark it on the floor before anything is cut.
Once it is located, there are usually two real options, and they should be priced side by side: open the slab at that one marked spot and repair the line, or abandon the run and reroute overhead through the attic and walls. A reroute costs more in pipe and patching but leaves the concrete alone. Which is better depends on the house, the number of failures already on record, and how much of the original line is the same age as the piece that just failed.