I recently spoke with a home owner who has had a number of plumbing issues, ultimately resulting in a new sewer line run from the residence to the main. The homeowner is concerned that there may still be a plumbing issue. After a recent bout of drain flies coupled with some strange smells, this possibility has come back to the surface.
During prior issues, a toilet was bubbling during other fixtures use, and some drains would periodically drain slowly until "a bubble would come up from the drain and things would drain at normal speed."
A snake and hose were apparently run down the vent stack (each vent) from the roof, but no blockages were encountered, and each one was flushed with "a lot" of water. (It sounds like at least 15 minutes of full flow from a 5/8" hose is how the water/flush was applied.)
Is there anything I can advise this homeowner in terms of testing for clogs or vent issues? In doing my own research I've also found instances of "dangling" lines in walls - and although not common, I believe this house (built in the late 70s/80s I think) has suffered from 5-10 years of "deferred maintenance", and I've heard the homeowner talk about buying it as a short-sale, so I'm trying not to write anything off, but also avoid the tin foil hat theories.
Note: one of the homeowners has apparently done rehab work with volunteer organisations before, and believes the smells to be suspicious, although I haven't heard this corroborated by the other homeowner.