Mine turned out to be the seal around the oil filler cap was not good enough. I think there is another thread with all the information. This didn't get picked up with a smoke test as it was not under pressure.
The most probable cause is a leak 'somewhere' on the vac system. Don't go changing expensive parts like mafs as mine had nothing wrong with it. All it improved was a bit of responsiveness as the old one had done 100k.
If you cannot spot an obvious fault on the pcv system look for any tell tale oil splatters as the oil vapour will escape under pressure.
Good luck