[arethuza]

It has already been used once in the UK:

https://www.theguardian.com/uk-news/2017/aug/05/met-police-f...

I would suspect if there has already been one public announcement then it will actually have been used covertly for quite a while.

[firmgently]

Another one which got a bit of press was Download (music festival) in 2015: "A police force has defended scanning the faces of 90,000 festival-goers this weekend and checking them against a list of wanted criminals across Europe."

http://www.bbc.co.uk/news/uk-england-leicestershire-33132199