[amoonki]

This is what I got from my googling:

Plastic parts are often made by pouring hot plastic into a mold. When the plastic is done drying, often a robotic arm or crane needs to pull it off to bring the plastic to the next stage of the manufacturing process. But it's hard to get the plastic off the surface once it's dried; it needs to be "popped" off of it. So manufacturers build ejector pins into the mold. When the plastic is done drying, the pins pop up and push the plastic off the mold, into the crane or whatever will take it to the next part of the operation.

[Aqua_Geek]

And when the pins hit the plastic part to pop it off, they often leave small, circular marks on it.