Cloud ceiling was overcast, approximately 3000', winds around 270 at 07 kts. Saw a lozenge-shaped object, dull grey in color, moving north to south in a straight trajectory. It was approximately 500'agl and was rotating very slowly (approx. 6 rpm)in a clockwise direction (viewed from below). I thought it might be a mylar balloon, but it was dull grey and maintained altitude, direction, speed, and rotation so precisely that I question if it was a balloon of any type. It did not follow surface wind direction, but I did not know the winds aloft at that time. There were no surface features or lights visible. The surface seemed very smooth, and the dimentions of the object were approximately 5' x 2' and shaped very much like a oblong cough lozenge. It rotated along it's longitudal axis parallel to the horizon. I am a pilot and a professional with years of experience. I observed the object for about 3 minutes as it traversed the visible sky, under the cloud bank.