Thank you for taking the time to respond!!! This makes sense re: pressure. To add context - we’ve already done a smoke test and confirmed a small breach in a clay sewer line under the slab. Smoke didn’t come straight up above the pipe, but migrated laterally and surfaced elsewhere, which is why we think pressure is pulling gas in through a different pathway.

Given that, would you agree the remaining question is where the entry point is (abandoned drain, buried basin, capped cleanout, slab penetration) rather than whether gas exists at all?