EDIT: I believe I found the answer, it's something turned by this: https://ctatools.com/products/5065
Found some trash on the street that i'd like to take apart, but this screw is in my way.
It's like a hex bolt, but with 5 sides, and rounded (a bit like flower petals).
Here's another picture from the top:
I tried turning it with various tools (hard to get pliars in unfortunately), no success so far. The material is very soft, which doesn't help..
yeah, it is inverted (or "external" torx according to your link). They also made sure to sink the screws into a whole where no pliars will fit...