I noticed a patch of land in shadow like condition with nothing obscuring the sun above.
Prior I was digging a tunnel under the patch. Back on the surface, I crossed the patch, not seeing the shadow. Returning and crossing the patch again, I noticed the shadow. Like being under a cloud or solareclipse.
edit 1
video, 27 Mb,
worldsave , 128Mb.
 edit 2, 3
I since noticed night mobs while I have been sleeping from sunset, a spider, creeper and skeleton on or coming from the patch. The spider repeated following me to the edge of the shadow.
edit 4
In the mean time I have built a scaffold tower in the patch. Climbing up, "my" head and surrounding scaffold became lit when they were at level 112, with feet at 111 in darkness. Only thing around at that level is the other scaffold tower, which was built south of the shadow patch and is totally lit by the sun. Also lit, 2 blocks north from the top of that lit tower, I had built an oak leave block, with vines growing.