There are plenty of signatures for life that we've been looking for and haven't found yet.
The obvious one is oxygen which is reactive so doesn't get replenished absent life... or, absent surface ice being ablated by solar and ionic winds, UPDATE 10/12 especially if sulfuric - like Europa does. Ooof. Then there's phosphine, or ammonia. Venus might have these. Venus also has volcanoes which eject all manner of nasty-smelling stuff up there.
Last week I got wind of nitrous oxide without nitrogen dioxide - if the planet is very watery, perhaps more so than Earth I admit. Today I'm reading about the Methyl Bromide.
It be all well-and-good; but as we've seen from Europa and from Venus, which are not habitable on their surface and in their clouds respectively, I should like assurances that these compounds in those concentrations cannot be produced by processes astronomic or geologic (again respectively).
UPDATE 10/25 Don Wilkins @Centauri.
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