This is the slogan of a post-9/11 (and ongoing) MTA campaign that's been borrowed by the Department of Homeland Security and many other agencies around the country and the world since.

I believe the original intent was mostly about reporting unattended bags that could contain bombs, not reporting your neighbor for hosting a socialist book club or whatever. Obviously it's vague and broad enough that it could be bent to more sinister purposes though.

No need for the neighbors to report subversive activity when everyone is already self-reporting all this information online, both publicly and in not-so-private private communications.

>"If you see something, say something"

Perhaps a slightly different context but I have heard this almost verbatim a few days ago on buses and trains in Birmingham, UK.

If I'm not mistaken too, I believe that "If you have nothing to hide, you have nothing to fear" is also something the Govt used a few years ago.

"See it, say it, sorted" is the current British Transport Police campaign:

http://www.btp.police.uk/latest_news/see_it_say_it_sorted_ne...

I think the current "If you see something, say something." is specifically targeted at child abuse. At least, that's how all the adverts make it sound.

Australia ran one of these campaigns recently: https://www.youtube.com/watch?v=FXxgclr7J9g

The content of the ad includes someone looking at someones trash, seeing some common chemicals and assuming they are making a bomb.

..and you still see this exact phrase on trains and buses in both the US and UK.

"If you see something, say nothing, and drink to forget"