Molding marks. Even Topres have them if you look hard enough.
My advice is to turn the keyboard around.
LoL!
To the OP-
Indeed they are just artifacts from the injection process. The vast majority of keys out there will have this, and it's usually on the back or bottom side. It shows up more on some than others, this can be in part due to the type of plastic, but more often it's the particular deformation stress when the sprue is released or separated.
So in other words... no worries =)