New Scientist: Acidic clouds of Venus could harbour life

Solar radiation and lightning should produce large quantities of carbon monoxide in the planet's atmosphere, but instead it is scarce, as if something is removing it. They also found hydrogen sulphide and sulphur dioxide. These two gases react with each other, and so are never normally found together unless something is producing them.

Even more mysterious is the presence of carbonyl sulphide. This gas is so difficult to produce inorganically that it is sometimes considered an unambiguous indicator of biological activity.