I was walking my dog behind a small shopping area parking lot within 300' of my residence. i heard a sound above and to the right. through a break in the overcast, i saw a large triangle shaped object with structural surface features and lights. it was moving slowly (forward i assume) at what could have been between 500' to 1500'. it suddenly began to disappear from the trailing end, the transparency revealing normal empty sky. my initial reaction was astonishment. i immediately began to question myself. did i really just see this? i am a private pilot of 30 years and am very familiar with just about all general aviation as well as commercial and military aircraft. i also know the typical low altitude flight and approach/departure paths locally. what i saw was outside of design and behavioral norms. the entire event lasted between two to three seconds.