What is the law on that sort of thing? I imagine it must be different in regards to who does it and what constitutes "evidence".
Wiretapping, eavesdropping, and recording all fall into this debate. Each state varies, usually depending on how many corrupt politicians were caught with various methods.
In most states you can record audio of any conversation you are part of in person (not over the phone) with few limitations.
Video is something that's not very well protected - there have been cases of people installing video cameras in other houses (no audio) and not getting prosecuted when caught because the law did not protect that privacy. (though they are usually successfully prosecuted for other things in connection with the video taping)
Those are changing and evolving over time, though.
Acorn operates these offices as public buildings (like a supermarket) so they can't sue for privacy. It'll very much depend on the local laws in each case.
Of course, in the USA acorn can sue them without good reason, and simply draw the case out so the people who did the film go bankrupt.
-Adam