The green is for zero emissions and also includes hydrogen vehicles. At the moment there are only hints it will lead to some benefits such as entry into clean air zones, but as stated current tec only recognises the letters/numbers on the reg plate.
I suspect the owner is parking in electric vehicle charging locations (which you can do with any plug in car) but like me gets sanctimonious knobs winging that it's taking up too much space or too long to charger for the 54 mile range it can do in ev mode... or someone nicked his engine.