Janus Cars is a local rental car agency in London which is seen in "The Great Game". It is part of the Ian Monkford puzzle given to Sherlock to solve by Jim Moriarty.
The clue is in the name, that of the Roman god of doorways, with two faces. Sherlock deduces that the company is a front and that they can arrange for people to disappear. He figures out that Janus cars smuggled Ian Monkford to Colombia, leaving a pint of donated blood on the seat of his rental car to make it look as if he has gone missing under strange circumstances.