There is no evidence of a warning letter over a new technique used for car theft in the U.K. The message has been in circulation since at least 2009.
Moreover, similar warnings have been shared by social media users since at least 2009 in several versions from different countries. In 2009, an email regarding a carjacking warning surfaced in New Zealand. In response, the New Zealand government issued a clarification about the hoax email. The email warned the recipients not to be on the lookout for a piece of paper stuck to the back window of their vehicle. The police asked the people to ignore the email about "supposed carjackings" and confirmed it to be a hoax. Regardless of the deceptive mail, police advised people to exercise caution and not exit a car with keys in the ignition.
We also found the same warning in a text image version from 2014 that circulated in Louisville. According to an article by WDRB, Major Mark Fox, from LMPD's 5 Division, said in a statement that the post about a carjacking scheme is fake.
Similarly, an identical letter starting with "warning from police" written in bold letters was also shared in 2020 on Facebook, this time in Canada. However, Paul Manaigre, spokesman for the Royal Canadian Mounted Police, told AFP by email in June 2020, "This is indeed a hoax and seems to surface every few years on social media. I can assure you we have not had any reports of this nature in Manitoba." AFP had debunked the claim.
Considering there are many identical posts on carjacking from previous years that have resurfaced, we mark this claim as false. There is no evidence of any carjacking crime that has occurred as a result of people sticking pieces of paper to car rear windows, and the police in the U.K have not issued such a warning.