A while ago, in a move to clamp down on illegal parking, the police invited civilians to join in on looking for offenders. Not just anybody can sign up to be a traffic warden though and you need to be working for one of the many incorporated companies that signed contracts with the police like the security and real estate management businesses.
Now that civilians are in on the act, you see very otaku looking like folks dressed in these green uniforms carrying around with them a terminal and a digital camera. They need to take a picture of the offending vehicle in order for them to get their commission I guess. You see them squatting to take a picture of the car with a smug face.
Photo taken at Akihabara some time last month.
I personally haven't been ticketed for illegal parking in Japan. The streets are so incredibly unclearly labeled that I'm not sure where I'm allowed to park so I always end up looking for coin parking.