I've walked through the metal detectors at a federal building while carrying, heard it go off, and instantly remembered that I forgot something in the car.
Went thru all of Carlsbad Caverns carrying my X9. Stood right next to an NPS LEO Ranger and chatted him up while waiting to enter the cave, which is only accessible thru a federal facility. Realized I was armed after I took my jacket off and felt my shirt tug on the grip. Passed six or seven more throughout the entire tour, then passed by another couple on my way out. Heaviest LEO presence I’ve ever seen in a National Park.
The post office is a nothing burger compared to that.