Our drain goes under our flower bed to the middle of the front yard. It is capped with a bubbler that will pop up and let the water flow into the yard. It could also be capped with a grate instead. It's possible the grass has grown over the grate or bubbler preventing it from letting the water discharge. When ours gets grown over my gardener (hubby) takes a screwdriver and taps to find it. Of course he knows the vicinity to look for it. Good luck.
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