Cotinine is an alkaloid found in tobacco and is also a metabolite of nicotine. Its presence in saliva, blood or urine is (ab)used as a presumption of nicotine consumption, whether by smoking tobacco products or by using smoking cessation products that contain nicotine (gum, patches, e-cigs,) and such testing is used to screen employees in companies that ban employees from smoking at all.
It can also be present in those who have had exposure to second hand smoke in levels that cannot be differentiated from that of a light smoker.
In Feb 2016, it was reported that in Belgium, people who failed a cotinine urine test in the previous six months would not be reimbursed the costs of a drug to help with pulmonary fibrosis, due the presumption that they'd been smoking. 
- Cotinine - Wikipedia
- Cotinine - Mondofacto
- Dietary nicotine: a source of urinary cotinine - PubMed
- Nicotine In Vegetables: 20 Pounds Of Eggplant Equivalent To 1 Cigarette - Huffington Post
- The nicotine contents of vegetables - Vidar's Index
- Belgium: A drug will not be refunded to nicotine users - PGVP