Time restriction is applied simply to remove "old" results that may not be relevant anymore, and to ensure this question is not made outdated by the passage of time or changes to varies countries laws.
Based on this article: http://news.bbc.co.uk/1/hi/7234786.stm
(Written 2006) Meanwhile, a Swiss national is serving a four-year jail term after three poppy seeds from a bread roll he ate at Heathrow airport were found on his clothes.
With similar cases more recently: https://www.buzzfeed.com/ishmaeldaro/canadian-woman-jailed-abu-dhabi
(Written 2016) Guards searched her bags and found a small amount of marijuana, no more than some "loose flakes," in her backpack, Augustyn's mother Joanne told BuzzFeed News.
Which countries are known publicly, to have charged somebody with a criminal offense for the possession of illicit substances based only on traces of the substance being found - between 2014 and 2019 (last 5 years from the question's post date)?
To be clear, a "trace" here would be an amount of substance that is detectible on a person, but by common sense could not have been used by the person any further. For example, the 3 poppy seeds above or traces of cocaine that are found on bank notes. Finding traces, that lead to a "real" find such as a "useable" quantity of e.g. cocaine would not count here.
Criminal charges in this question means having been charged with committing the crime, whether the case fell through in court or not. The person did not need to be found guilty to apply.
Also the "illegal substances" in question; are anything the country deems to be illegal. The important aspect is that only a trace-amount was found.
Please ensure any given answer has at least one example case given for each country. This question requires substantiated information, as such speculation or gut-feel is not sufficient.
