They are a "hm, that's interesting" kind of a signal. They simply mean that a simplistic chemical view of the atmosphere does not explain the observation. It could mean the presence of life, or it could mean some other unknown, inorganic process.

See for example the Viking experiments: they detected signs of life on Mars, only to be disproved later by better understanding of soil chemistry.

https://en.wikipedia.org/wiki/Viking_lander_biological_exper...