I went out and saw this cigar shaped object, high above, about where the sun would be at 12 o'clock. I stared at it trying to identify it, but could not. It was moving slowly westbound, at (my estimate)about 25,000 feet. I went in to get my binoculars, but when I came out, it was gone.As I mentioned in my summary, this object was not something that I could identify. It seem to have some metallic components because it did reflect the sunlight. It also had a strange movement, like drifting and yawing at the same time. What amazes me the most is that it was not there when I came back, about a minute after. At the pace it was moving, it should have been there still. I immediately called the San Juan Flight Services Station, to find out if they had seen something in the vicinity with their primary radar. They offered me your number instead. I place a call and left a message in the answering machine, but I erroneously reported the time of the event as 1605Z, instead of 1805 which is correct. I later called the San Juan Center (CERAP) to check if they had any information that could confirm or explain my sighteen, but they said it was difficult to check their recordings and data bases and to get something usefull from there. One more thing, the object was in the vicinity of the Dorado Beacon (DDP) which is an NDB used as reference for IFR flights inbound to Puerto Rico. I am a private pilot and I can tell you, what I saw was not an airplane or a rotorcraft.