We were sitting on the couch watching tv at night. Its an overcast night with cloud cover that has a glow to it from the lights below. We both saw at the same time a black triangle (I think maybe a diamond- we weren't seeing it straight on) fly on a straight line at a descending angle left to right across the window. We noticed it in our periphery vision at first- it was such a dark clearly geometric shape moving against the glowing sky. it disappeared over the tree line. I stood up to watch it as it passed and saw it fly out of sight past the tree line. Thats when my partner said, "did you just see that too?" So I knew I wasn't seeing things. We were both surprised. It had no lights and was fairly discreet against the dim sky. it was totally silent and faster and lower than a plane. It crossed into and out of sight in 6 seconds or less. I have no idea what it is, I have been trying to figure out if it was some kind of military drone? or other kind of UFO. Whatever it was it was very unusual, we see a lot of planes fly by but they are much further away with lights and slower. I have been trying to find out more about what it could have been ever since.