A company can't post a §30.06 sign in a parking lot and enforce it against an employee. It doesn't matter that it's trespass law or a criminal statute versus company policy, since §30.06 doesn't apply to people who have "effective consent" - §52.061 basically forces a company to grant its employees effective consent for trespass purposes and would give you a cause for civil action if they pushed the matter.§52.061. RESTRICTION ON PROHIBITING EMPLOYEE ACCESS TO OR STORAGE OF FIREARM OR AMMUNITION. A public or private employer may not prohibit an employee who holds a license to carry a concealed handgun under Subchapter H, Chapter 411, Government Code, who otherwise lawfully possesses a firearm, or who lawfully possesses ammunition from transporting or storing a firearm or ammunition the employee is authorized by law to possess in a locked, privately owned motor vehicle in a parking lot, parking garage, or other parking area the employer provides for employees.
IANAL!