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