[burgerbrain]

I am under the impression that public libraries in the US will often place signs in their windows saying something to the effect of "The FBI have NOT yet visited us". When they get served with a warrant and a gag order, they simply remove the sign (but are unable to actually mention anything).

[biafra]

That sounds interesting. Do you have a(n online) source for this?

Although I think removing that sign would be considered violating the gag order.

[JoachimSchipper]

rsync.net famously does (did?) the same thing, publishing weekly messages of "as of april 11, 2011 we have not been served". There was some debate about whether they could stop publishing these messages without violating the gag order.