Dark triangular form appeared, flew at a steady pace a few blocks over the treeline in Greeley, Colorado and then dissapeared.I was sitting on the front porch steps of my sister's house smoking a cigarette and staring up at the constellation Orion; that's probably the only reason why I noticed it. I looked down and it appeared just beneath Orion as if it just winked into existence. It was just above the tree lines and very large, I can not judge distance and size very well, so this is just as close as I can figure... Above the tree tops but way too low to be at helicopter height or airplane height. It was just above a 12 story tall building. I hope you understand this, I'm still really shaken by it. Freaked. All of the above, I've never ever seen anything like this before.Anyways, it was a perfect triangular shape (the lights from the street illuminated it slightly and it was darker than the night sky behind it), black in color, with three very large lights on each tip of the triangle. It moved at a very steady pace that I could easily follow with my eyes, above the tree line, and for about two-three blocks down the street. The lights were very, very dimmed... like they were on the lowest they could go without being turned completely out but they were illuminating absolutely nothing. the lights looked like they were slightly bulbous and not streamlined with the rest of the .. craft? It flew straight down the street, didn't spin, nor did the lights come on and the entire thing, during the entire time it was completely and utterly silent. I could hear the highway in the background, cars a few streets over but it was like someone pushed a mute button on this thing.I watched it fly down the couple of blocks and then it just went completely black again. Just gone.Also, about 3-4 minutes later, both my sister and I witnessed two helicopters in the vicinity; one flying toward where the object disappeared and one that was circling around the area. It was too dark to tell what kind of helicopters they were. I did not see them again.Both my sister and brother-in-law have seen this same object. One just the other night on highway 85 from Denver, Colorado and my sister saw the same thing on the same occasion.I'm going to go out tomorrow and sketch the area from the point of view where I was sitting and draw in the object how I saw it from that point. Maybe that will help determine the size.My background? I do paranormal investigations. I can handle ghosts, intelligent and residual, nature spirits, poltergeist activity and even the nastier things that go bump in the night... but I was always afraid of 2 things that get lumped into the paranormal category and one of them is UFOs so I never study them. I was much happier when I was in denial, thank you very much.Directly afterward I called my friend, who is a paranormal investigator and reporter and left him a voice mail message on his cellphone (freaking out) telling him to call me right away. About 15 minutes later he called me and told me that he thinks I saw "what the British call a Silent Vulcan" and to go to this website and file a report with Peter Davenport. And since he's into the UFO side of things and I'm not, I listened to him.