Ordinance they reference in the sign:
[...]With respect to dangerous weapons (other than the firearms that are prohibited under above Subsection A), and pursuant to the police powers of the City of Burlington, no person carrying a dangerous weapon may enter and/or remain in any part of a building that is owned and/or occupied and/or controlled by the City of Burlington. This prohibition against carrying a dangerous weapon in a municipal building applies to both open carry and concealed carry of a dangerous weapon.
Pursuant to the provisions of § 943.13(2), Wis. Stats., the City of Burlington shall post a sign that is located in a prominent place near all of the entrances to the municipal buildings subject to this chapter, in a manner that any individual entering the municipal building(s) can be reasonably expected to see the sign. Each such sign shall:
Have dimensions of at least five inches by seven inches; and
State that "No person may enter or remain in this building while carrying, whether openly or concealed, a firearm or dangerous weapon. Section 943.13, Wis. Stats., and Ch. 308, Burlington Code of Ordinances."