UPDATE: the light pattern exactly matches the landing and nav lights of a Boeing 727
The Nashua image [UPDATE: actually not taken in Nashua, just used as illustration] was presented as a "Boomerang shaped UFO", yet it appears to be the landing lights and navigation lights of an ordinary plane. The most obvious indicator of this is the red and green lights that are at the airplane wing tips. Green on the plane's right (starboard) wing (viewer's left), and red on the left (port) wing. Here, for comparison, is the landing and nav lights of an A380:
This set me thinking, there must be many instances in which landing lights are mistaken for UFOs. A plane approaching the viewer from nearly head on can appear to be moving very slowly in the sky, and then then suddenly speed up as it passes. With it being dark, it's hard to see much beyond the lights, so the brain fills in the gaps, and you get a "triangle" or "boomerang" shape.
Perhaps it might be useful to try to gather images of the landing light configurations of various jets, so they could be more quickly identified? What plane, for example, has four landing lights like in the first image?
There are surprising few images that are easily findable, so by collecting them in one place, it might save a lot of time, and misunderstanding.
Landing lights are always on when landing, so the photo need not be at night. A collection of photos taken from a good viewpoint at a bust airport would quickly give a good reference base.
Here's a good video showing several planes passing very low overhead. Because it's low, over bright city lights, and misty, you see a lot more of the plane than you would on a dark clear night away from the city.
Here's a plane flying nearly directly towards the camera, seems to be hovering for a while.
And more planes landing
Interesting perfect triangle here appearing as a reflection.
These landing vides are maybe not the perfect reference though, ideally you'd have a shot zoomed in on the plane while it's still at quite a distance. Maybe something like this, where at night, through a zoom lens or binoculars, a triangle shaped UFO might be seen to slowly sink below the trees and vanish.