As you can see in the above picture, it is understandable why some people might think that the object is a UFO. It even appears to have a dome shape on the top of it. Lenticularis (Latin for 'lens-shaped'), or lenticular clouds, have three different types; stratocumulus, altocumulus, and cirrocumulus. They can occur at altitudes up to 40,000 ft., and can even sometimes precipitate rain. They are formed by a mechanical obstruction of the ambient wind flow, which could be hills, mountains or even large buildings. When stable, moist air flows over mountains, it can create long-standing waves on the downwind side, and if the temperature at the crest of the wave lowers to the dewpoint, a lenticular cloud may form. I would highly encourage any Field Investigator to spend some time Googling pictures of lenticular clouds, so you can be familiar with their appearance...and because most of the pictures of them are truly magnificent!
The next anomaly I'd like to introduce is called a 'Fallstreak Hole'. This amazing anomaly is the one responsible for people commenting, "it looks like a cloaked mother ship!" I must admit, it does remind me of the 'Predator' just a tiny bit.
There are many conditions that can cause clouds to take on shapes we not understand. Pareidolia is the scientific name for a common phenomenon...seeing familiar shapes in objects such as clouds. I would strongly recommend that any cases involving anything that even remotely looks like it could be a cloud be evaluated thoroughly. More than likely that is exactly what it is.