The September 2020 announcement by Greaves and colleagues of a tentative detection of phosphine (PH3) at ≈20 ppb in the cloud deck of Venus reopened, with unusual force, the question of whether the most chemically reduced terrestrial analogue of a planetary atmosphere can be reconciled with abiotic explanations. Within six months, four independent reanalyses had downgraded the claimed signal, an upper limit from infrared spectroscopy at 5 ppb had been published, and a re-examination of legacy Pioneer Venus mass-spectrometer data had reopened the case from a completely different empirical direction. In 2021 and 2022 the dispute spread to ammonia (NH3), to the photochemistry of phosphorus-bearing species in concentrated sulfuric acid clouds, and to the question of whether mantle-plume volcanism could deliver phosphides in sufficient quantity to mimic a biological signal. Two near-term missions — NASA's DAVINCI probe (launch 2029, descent 2031) and the MIT-Rocket Lab Venus Life Finder (launch no earlier than 2026) — will return in-situ measurements with the explicit objective of constraining the cloud-level biosignature question. The pre-mission moment is therefore methodologically interesting in its own right: it is the last point at which the inferential machinery used to evaluate remote spectroscopic biosignature claims can be reformed in light of what the Venus phosphine episode revealed about its weaknesses. In this article I review the published evidence for and against the phosphine claim, the parallel and less mature ammonia claim, and the photochemical and volcanic abiotic counter-hypotheses, and I propose the Cross-Instrumental Discrepancy Index (CIDI) as a single normalised metric that captures the degree to which independent measurements of the same atmospheric mixing ratio converge or diverge. CIDI, applied to the post-2020 Venus PH3 dataset, returns a value of approximately 0.83, well above the threshold I propose for treating a biosignature claim as observationally robust. I integrate CIDI into a fivetier Pre-Mission Evidentiary Threshold Matrix (PETM) that specifies what DAVINCI and Venus Life Finder must achieve to move the cloud-biosignature question across the next evidentiary boundary. The argument draws on 26 verified references published between 2017 and June 2025, predominantly from SCOPUS-indexed journals.