in that position it is very likely a leaking waterpipe or drain.
I'd look at the waterpipe first.
Find the outside stopcock, which will be in a pit next to where the front gate used to be when the house was built, probably in a straight line to where the kitchen sink used to be when the house was built (or it might go under the side alley if there is one. And look for the indoor stopcock, where the pipe come out through the floor under where the kitchen sink used to be when the house was built.
Either or both of these might have been hidden under flowerbeds, paving slabs or concrete for your convenience. There might be a tap or watermeter in the pavement under a small iron or black plastic trapdoor.
Get a young person with good hearing to stand in the kitchen late at night when all is quiet, and turn off the outside one, then on again, and repeat. If they notice faint white noise stopping and starting, you have a leaking supply pipe.
If you have no young person you can buy an engineers stethoscope for a few pounds and press the tip against the cold pipe or tap. You may not notice the faint hiss of a leak, but you'll notice when it stops and starts.
Then look at the drains. There will be a soil pipe going into the ground, possibly cast iron; and a rainwater downpipe, possibly going into a gulley, and a gulley for the kitchen sink, possibly brown glazed pottery (clay)
All of these are probably cracked or broken and leaking in the ground. Clues are: Patched, repaired or cracked and sunken concrete, paving or ground around them; wild tomato plants; damp, rich soil with bright red worms in it.
Take some photos of all these suspects please.