UFO or meteorological phenomenon?
I've spent some time looking at the skies over the Dutch countryside, and think your UFO has an earthly explanation. Sometimes clouds consist of (at least) three layers, each distinct in appearance, moving at different speeds and altitudes. As a result the Sun may hit them differently too, causing for instance one layer to intensely reflect light, making cloudshapes in the next appear very dark.
A man of Trachis' warning:
"When the Barbarians discharge their arrows they obscure the light of the sun by the multitude of the arrows"
Dienekes of Sparta's reply:
"Good news, for if the Medes obscure the light of the sun, the battle against them will be in the shade"
~Dienekes, Battle of Thermopylae 480 BCE [Herodotus 7.226]