Light or small object, drifting slowly E-W on a clear afternoon. No sudden maneuvers, flew v. slowly.I observed a possible UAP (UFO) over S. Boulder, Colorado about 1415 MDT (GMT - 6) on 14 Mar 09. I was out enjoying the sun when I noticed a light or silvery object (a point) drifting very slowly to the W. It passed overhead about 1415 and finally disappeared behind the trees. It did not perform any of the sudden maneuvers often reported of UAPs, such as darting off at extreme speed; it just slowly traversed the sky. Size, height, speed unknown. It left no trail, did not flash, etc.A star or planet, or even a spacecraft such as the ISS? Not likely. It was too bright to be the former; and S/C are usually launched to the E. to take advantage of the Earth's rotation, so fly W-E. They also usually go much faster, in angular velocity. Not a jet; it went too slowly.Weather balloon? Possibly. Wind here has been light NE all day (as usual); but winds are usually W. at higher altitudes. It did not seem to slow down or get fainter as it would if it had been climbing. It could also have been a UAV (pilotless drone airplane), very high or flying very slowly. It jogged to the N. a bit then continued drifting W.Location observed from: ~Lat. 40° N., Long. 105° 15' W. (just N. of Baseline Road near 29th Street in S. Boulder), el. ~1625 m. (5330'). Weather: clear, light NE wind, 15° C. (59° F.).My background: A.B. in Applied Physics, MSEE in weather radar.