Walking the dogs with my girlfriend after dinner, noticed an unusual light pattern in the northern sky heading over sr 50 and mariner blvd area. took me a few seconds to realize i was looking at something abnormal. i looked for red beacons or directional green/red lighting on the craft and observed none. i hold a certification in airplane mechanics and noticed this crafts light pattern was unusual. unfortunately the sky was dark and i was unable to see a defined outline of the craft, there were several pale white lights which appeared on the underside of the craft in the shape of a triangle. see attached sketch. it was quiet enough out to hear a dog barking several blocks away and i did not hear any sounds coming from the sky, i questioned if it could be a drone but it appeared too large for something you could buy, and it was windy out with a very light drizzle. unfavorable conditions for hobby flying. the craft seemed to be traveling at or just under cloud level to possibly maintain obscurity? appeared to have faster than normal movement compared to usual air traffic. craft made a sharp left hand pitch, then appeared to level off and move up into cloud cover. observation time was short approximately 10-15 seconds. she asked what that was, to which i replied i was not sure. she asked again a bit more seriously but neither of us were sure what we just saw.