Sewage smell is coming from the rooftop vent but?

I smell sewage in the front yard. I called the city and they checked their end. My friends said if my plumbing inside the house isn't back up then its probably not rooftop vent. Is that true?If I stand on the ground directly below the roof that where I smell it.If I walk away from that spot the smell stops. There are no puddle or wet spots in the yard. I waiting for the neighbor to get home with the latter but I would rather not go on the roof if I can rule it out.