UFO over Amsterdam, what's your theory?

Seeing all the pics from SteemFest reminded me of something strange I once experienced in Amsterdam.

Surely many people experience and see strange things in Amsterdam, but this one comes with a photo too.

I was casually snapping pics around the city in 2008 and didn't actually see anything out of the ordinary till I got home and was editing my photos.

I pulled this one up in Lightroom and noticed something odd.

See anything out of the ordinary?

This is a closer cropped view of the top right corner:

Here's a zoom even closer :

Looks to me like a classic UFO saucer shape.

btw the EXIF info for the shot was :
Canon 40D | 17mm | 1/640" | f/5.6 | ISO 320

Let me know what you think in the comments.