Smell in living room - could it be leaking pipes?

We've been noticing a sewage-y smell that comes and goes in the living room. Our bathroom is at the back of of our flat and the living room is at the front. I have a photo of the mains drainage for the building and I don't see any lines under the building that would indicate pipes but I know nothing about plumbing.
To try to explain the picture. At the front of the property near the living room there is a RP written in blue that attached to our flat. The line for the pipes runs parallel with the property but isn't under it as it connects to others. At the back of the property there is a W.RP in blue and this links with a SP in red and they seem to have the same access point and the red line runs along the back of the property and connects with other properties.
Could we have personal pipes underneath or floors?

Advice from 1 local tradesperson

Hi!
Its quite possible there is personal drainage under your boards. It would suggest a leak as the traps on the drainage pipes should emilinate any smells
regards
craig

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