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I'd advise against carrying in that building. Odds are, they've got plainclothes officers walking about as well as uniformed officers. I'd not want to run the risk of being "made" and the resultant fuss.

Additionally, as Barres pointed out, the law defines "premises" as "a building or a portion of a building." If the locals have decided that the entire building is a courthouse, you'll have a bit of a sticky situation on your hands. You might technically be in the right, but a jury might not believe that. It seems to be quite the gray area, and not one into which I'd want to venture.

As for the sign being "verbal notice," well, that's nonsense. What the sign is doing is informing you of the local constabulatory's decision that the entire building is off-limits. The sign isn't the restrictive force here; it's only the messenger. (Similar to any sign outside a courtroom proper that declares that firearms are prohibited within. The firearms are prohibited not by the sign, but by the court which ordered the sign put up.)

(I hope I'm reading the law right; I'm definitely no lawyer)