This fascinating poster is currently being used in London by the Metropolitan police. If you click on the image you should just about be able to read the rather interestingly phrased smaller text. Just what is 'odd' behaviour? It seems from the poster's text that you inform the authorities and then THEY decide.
During my HND, a friend of mine called Martin, photographed the location of various CCTV cameras/security systems that were springing up in Milton Keynes. The project's aim was to show the growing use of electronic surveillance in the urban areas - most of it operating without any public awareness that it was there. Undertake a photography project like that in London and you may find yourself trying to explain your college work to an anti-terrorism officer.
The problem for many professional and amateur photographers is how to prove that you ARE a legitimate photographer, interested in creating images when and where you see them. Could you prove that you were just photographing something because it was your job or hobby? I've been in professional photography seven years and carry virtually no official identification for who I am, who I work for or what I do - unless you count a driving license and a business card as ID. In light of the current security issues, I'm looking into rectifying that..... if I can !