In Lin et al. (2014), an interesting possibility of using biosignature detection to infer the presence of not just life, but intelligent life, is explored.
One way you can infer the presence of life on a planet is to look at the atmospheric ratios between compounds, elements, or isotopes in an atmosphere and find that they are out of equilibrium. For example, if you found molecular oxygen in combination with a reducing gas, there would be a readily available way for life to generate energy by harnessing the changing energy by combining oxygen with the reducing gas.
But what if you wanted to use atmospheric biosignatures, to find ETI instead of plain old dumb life?
It has been suggested that we could look for signs of pollution in exoplanet atmospheres to guess at the presence of ETI. While high concentrations of molecules such as methane (CH4) and nitrous oxide (N2O) can be suggestive of polluting life, they can also be created by unintelligent sources. While finding weird amounts of CO2 can be explained away, looking at more exotic compounds (namely specific chlorofluorocarbons) can provide much stronger evidence that life exists. Not only are they only significantly produced by unnatural processes, but some of them have short lifetimes, which could constrain how recently the ETI was on that planet.
What is extra cool about these molecules is that in high concentrations (~10x what we have here on Earth), they should be detectable with ~1 day of JWST time (RIP early 2019 launch date) for a planet around a 6000K white dwarf.
If one wanted to be silly, they could suggest using these unnatural CFCs as a form of METI beacon to announce our polluting presence to the galactic club. Not that they’d want such a self-destructive member.