Nitrogen trifluoride (NF3) can be called the missing greenhouse gas: It is a synthetic chemical produced in industrial quantities; it is not included in the Kyoto basket of greenhouse gases or in national reporting under the United Nations Framework Convention on Climate Change (UNFCCC); and there are no observations documenting its atmospheric abundance.
Links:
[1] http://admin.indiaenvironmentportal.org.in/feature-article/nf3-greenhouse-gas-missing-kyoto
[2] http://admin.indiaenvironmentportal.org.in/category/author/michael-j-prather
[3] http://admin.indiaenvironmentportal.org.in/category/author/juno-hsu
[4] http://admin.indiaenvironmentportal.org.in/category/journal/geophysical-research-letters
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate-change
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/green-house-gases
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/kyoto-protocol