I was working as a security officer at the Feast Of Portland. I stood at the main gate all day. Around 5pm I happened to look up and saw what I can only describe as a "classic" disk shaped object headed from the south and flying over the Ross Island Bridge. I was practically under the bridge so I didn't see it until it came over the bridge. It then turned south and headed towards SE Portland. It made no sound. It looked like it was around 15-20 feet across and about 10 feet tall. It then was about a half of a mile away from me over SE when it stopped for a second then started making left and right movements. It swung left to right then left again pretty quickly kind of like it was on a pendulum. I started to question whether was a drone or something like an aircraft at this point. I wanted to take my phone out to shoot a video but my supervisor was somewhere nearby and we aren't allowed to use our personal phones. It made the quick maneuvers left to right about 5 times then all if a sudden it stopped moving then simply vanished! I got chills down my back just then because I realized that it definitely wasn't a drone or something manmade. It completely dissapeared. I had a very clear view of it when it dissapeared and I can't explain how it did what it did. One second it was there and the next it was gone. It definitely didn't take off quickly or fly away. I felt that I couldn't tell my coworkers or supervisor because they would have thought I was crazy so I didnt talk about it until I came home and told my wife the story. My heart was racing for quite a while after it happened. The whole rest of my shift I kept looking up into the sky looking for it or anything else out of the ordinary. I've seen lights before at night and also I've witnessed black orbs flying overhead during the day but i have never witnessed something move or disappear like that before. I felt like I have seen something very special which is why i have decided to make this report.