Waste-water underground leak?

Ive got some very soggy spots in my lawn. I checked the water meter and the leak detector was not turning. The city checked the meter and said it was okay. I use city water/sewer. I have started to wonder what other sources could contribute to this. I have checked in my crawl space and have seen no leaking water in nay pipes there. I thought maybe it could be an underground leak in my waste-water discharge line towards the sewer. How would I detect this? Wouldnt it smell? Thanks for anyones help
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