A Trash Can lid
Car Bumber #1
Car Bumber #2
They look like flow marks but aren't. These are photos from various polyolefin materials around my house that have been exposed to the sun for a number of years. The additive package designed to prevent this has been consumed and the UV photons are now having a free-for-all. The changes are largely cosmetic, but will eventually become structural.
I'm not sure why the variation exists, but in an almost contradictory statement, I think it could be related to the flow in the mold. I'm hypothesizing that these ARE basically a flow mark, but the marks could not be seen because human eyes are not sensitive enough to detect the original defect. Now after 10 years in the sun...
To further support this idea, I've worked with compression molded PP parts that were also strongly oxidized, but that oxidation was much more uniform than in these injection molded parts.