2 holes exactly opposite seems normal. but I don't think they vent and yours has been drilled in the inside as well. It is possibly a solution to avoid a different hissing sound. I put a coin that if you block theese holes you will have a hissing sound (but different) at idle when hot.
If yes, then (according to 3200 science gathered here and there...) : it's because the vacuum created at idle is too important, creating a hissing sound that supposedly comes from the crankshaft gasket as the vacuum takes the thin oil film a bit away.. Quite scary. The solution (or one solution) is to limit or block the vacuum at the one way valve in copper going from the seperator to the intake. According to some readings, some Maserati techs did that in germany with a little screw in the one way valve, others (like me and other owners in France) did it with a part of a rivet in order to keep a vacuum but reduced.
If I'm right then the holes in your cap would be a different solution to this problem. But IMHO it is not the best as exterior air in the crankcase bring also humidity, water...
You may want to take a look at this topic. I tried to make a "research" on the crankcase ventilation, there are some mistakes, but you can see the rivet photo..