At the building site- OHSA safety regulations state that workers must use all applicable safety gear and that includes hardhats.
Tony and McGee use a small, portable ground-penetrating-radar-type device to look "into" the concrete support column at the crime scene. Initially, the image on the screen has a passing resemblance to actual GPR output. But then a ghostly face fades clearly into view amongst the GPR readings, which is very far removed from the actual capabilities of such a device.
At 41 Seconds when the construction worker finds the dead body, the actor playing the dead corpse can be seen blinking twice in rapid succession.
The NCIS team visit two mobile home parks, one in Alexandria and one in Arlington. The ones seen are rural and have mountains in the background. Both Virginia localities are suburbs of Washington, DC, a very urban and non-mountainous region.
Despite the address being on Harrison Lane, the X on the map is on the corner of Robert E. Lee Drive and Groveton Gardens Road.