Yep - know of this and seen them "in action" (hiding behind a wall with a laser gun) Like vigilantes.
An extract from a local rag:
"When they catch someone travelling at excessive speeds – and not 'only just' over the limit but a fair way over it – they send the licence number to the police, who send out a warning letter.
If it happens again they get a second letter but this time, and here's the clever part, they're told that if they are then caught speeding thereafter they will get prosecuted instantly and NOT just warned."
Unfortunately we can expect to be targeted as the individuals who offer themselves up for this 'work' will inevitably target cars they perceive as being 'dangerous '. Jealousy, prejudice are words that spring to mind in their motivation.