Location: middle of historic hudson, wi, several blocks from the st. croix river in a residential area. late saturday night. i was about to get into my car in the parking lot of a building. the car was pointed south with the building (roughly 4 stories) behind us across the parking lot. i was on the drivers side, my fiance was on the passenger side. i was facing south, she faced north. i was about to open the door when my fiance says "wow, what is that?!?!" and i turn around just in time to see a large, brownish, object disappear behind the building. it moved west above a street and quickly moved out of sight behind the building. it was illuminated though i could not see if it was reflecting or providing its own light. it seemed like it had undefined edges and moved quickly, i approximate around 25 mph to conceal itself behind the building. the weirdest thing was how it moved: best described like how a piece of paper or a leaf moves when it is lifted by heat from a fire. gliding. i had no idea what it was when i first saw it. not like anything i have ever seen before or would have expected to see from a ufo sighting. i heard no sound from it. completely silent. i barely glimpsed it, though my fiance saw it for several more seconds than i did. it was not natural. it was much to large to be a bird - around the size of a sedan. it glided and had an undefined shape. it was above the treetops. we immediately ran around the building to try to spot it again but did not see anything. we stayed in the area a while longer and looked on roof tops and around the block but saw nothing. while walking around one corner of the building i felt my hair rise and i got goosebumps but saw nothing. just a very disturbing feeling. i returned to where i was when i saw it and waited for cars to drive by to see if maybe it was just their lights on the trees. it definitely was not. nothing else to report.