I seriously think they breed in the forest around Stockholm - some are from permanent courses, although these are changed around every year. At one control site I found no less than 5 different flags of varying types!
And yeah, the track network vanishes every time you ride/run over some bare rock...I think you have to look for the painted markings on the trees?
Yes there were two different types of hanging flags, flat plates and permanent looking controls too.
I think some of the tracks were just random tracks. The actual MTB trails I found today seemed to have painted trees, at least the marked trails did - I think Trailforks shows a lot of unsanctioned trails, some of which I really don't want to be riding on (especially not this close to WMMTBOC).