Astronomy/In the sky...
I stepped outside about an hour ago, 10:30 pm-ish, to have a smoke. While I was looking up at the stars as I always do, I noticed a horizontal line in the sky, it was obviously really dark but the moon was very bright, the "line" seemed to be lit but I am assuming that was some kind of reflection of moon light. I thought nothing of it until I noticed that it was moving. It looked like the trail from a plane but I couldn't find either end, it went straight across the sky horizontally, perfectly horizontally from my location, yet it was moving over my head, from behind me to in front of me fast enough that within about 5 minutes, I couldn't see it anymore. I pulled my husband outside before it disappeared over the horizon because I just couldn't figure out what it was. He said it was the trail from a plane and it appeared to be moving like that because of the Earth's rotation. Is that correct? I have just never seem such a thing. If it was the trail from a plane it was by far the longest one I've ever seen! Id also like to add that it stayed perfectly staight the entire time. I hope my explanation was clear enough for you to get an understanding of what I saw. I look forward to a response!
I think your first suggestion is probably correct. These jet contrails can be quite amazing--very staight for a long period of time. And with the cold weather, the last even longer. The jet stream can carry these across the sky quite quickly, and that would explain what you saw.
Next time, try taking a picture. Even if it doesn't come out perfectly, we can often learn a lot from the faint traces that are visible !