While fishing at Lake Alpine, I saw what I thought to be a silver aricraft, possibly a Cessna 140 or an Ercoupe, as these planes often are not painted, but are polished aluminum. I couldn't discern an aircraft shape per se, just a shing object that appeared to be 8 to 10 miles to the south. It was traveling at a pace one would expect of a small plane, and appeared to be 5,000 agl. (Lake Alpine is 7,300.) I watched it travel south for 10 or 15 seconds and suddenly it abruptly changed directions (now S.W.) and shot out of sight in a very short time, two or three seconds. There was no sound. This is the fourth (similar) incident I have seen here in Calaveras County since moving here nearly four years ago, but this is my first report