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18 U.S.C.
United States Code, 2011 Edition
Title 18 - CRIMES AND CRIMINAL PROCEDURE
PART I - CRIMES
CHAPTER 44 - FIREARMS
Sec. 930 - Possession of firearms and dangerous weapons in Federal facilities
From the U.S. Government Printing Office, http://www.gpo.gov
§930. Possession of firearms and dangerous weapons in Federal facilities
......
(h) Notice of the provisions of subsections (a) and (b) shall be posted conspicuously at each public entrance to each Federal facility, and notice of subsection (e) shall be posted conspicuously at each public entrance to each Federal court facility, and no person shall be convicted of an offense under subsection (a) or (e) with respect to a Federal facility if such notice is not so posted at such facility, unless such person had actual notice of subsection (a) or (e), as the case may be.

I once asked a Federal Protective Service officer what the underlined part meant to him and his reply was "have you ever heard that a federal building was off limits? That's good enough." My thought was, prove that I have.

But it's actually 39 C.F.R. § 232.1(l ), which covers the post office:

Weapons and explosives. Notwithstanding the provisions of any other law, rule or regulation, no person while on postal property may carry firearms, other dangerous or deadly weapons, or explosives, either openly or concealed, or store the same on postal property, except for official purposes.

Nothing in 232 requires signage that I've seen.