This one is very strange as it moves different than the other clouds and changes shape quickly. In 2011 a daytime UFO came overhead in Oregon less than 500 feet and could see it as an energetic movement in a rough shape of a large disk. (I described this in a past thread as shimmery particles, like the 1970's star trek episodes of the transporter look) Im pretty convinced based on that sighting that they do this type of stealth behaviour to hide from view. When I witnessed the shimmery cloud there were no other clouds in the sky and probably would not have detected it but closeness overhead provided the opportunity.