I think I asked this last winter.  

Is there a reference one could access to know?

I would assume not past tsa screening, and anywhere else would depend on who owns the airport and whether or not they have it posted. My local airport is municipal owned but not posted. The local car rentals are all there as well so this might have impacted their decision to post or not.


edit to add, check    WI state statute 175.60(16)(a)(8)





(16) Prohibited activity.

(a) Except as provided in par. (b), neither a licensee nor an out-of-state licensee may knowingly carry a concealed weapon, a weapon that is not concealed, or a firearm that is not a weapon in any of the following places:

1. Any portion of a building that is a police station, sheriff's office, state patrol station, or the office of a division of criminal investigation special agent of the department.

2. Any portion of a building that is a prison, jail, house of correction, or secured correctional facility.

3. The facility established under s. 46.055.

4. The center established under s. 46.056.

5. Any secured unit or secured portion of a mental health institute under s. 51.05, including a facility designated as the Maximum Security Facility at Mendota Mental Health Institute.

6. Any portion of a building that is a county, state, or federal courthouse.

7. Any portion of a building that is a municipal courtroom if court is in session.

8. A place beyond a security checkpoint in an airport.

The airport is posted now.  I flew south for spring break and noticed the stickers on the doors by the departure drop off.

Yep, the red doors that separate the parking structure from the 2nd floor walkways are posted too.  I think the law covers you for parking lots, but I'm not completely sure about that either.  My brain keeps going back to recent post 9/11 when they had TSA or Army NG checking everyone's car at the entry points for the parking structure.

I think all doors at Milwaukee airport are marked. Not just parking garage.

ATW and MKE are posted for sure. I think GRB is posted too.