TexasCHLforum.com

Sec. 46.03. PLACES WEAPONS PROHIBITED. (a) A person commits an offense if the person intentionally, knowingly, or recklessly possesses or goes with a firearm, illegal knife, club, or prohibited weapon listed in Section 46.05(a):
(1) on the physical premises of a school or educational institution, any grounds or building on which an activity sponsored by a school or educational institution is being conducted, or a passenger transportation vehicle of a school or educational institution, whether the school or educational institution is public or private, unless pursuant to written regulations or written authorization of the institution;
(2) on the premises of a polling place on the day of an election or while early voting is in progress;
(3) on the premises of any government court or offices utilized by the court, unless pursuant to written regulations or written authorization of the court;
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Since you mentioned that the building houses some portions of the court, I think TPC 46.03(3) is what they are using for justification. It says that "premises has the meaning assigned in 46.035, which is ""Premises" means a building or a portion of a building."

To me it seems unreasonable to restrict the entire building if the layout would allow them to move the metal detectors to a different location and still "protect" the court... but just because it is unreasonable doesn't mean they can't do it.