I've got an 03 WRX. Had it since new. About a year ago I got stuck in some really slow stop-n-go traffic. The engine usually idles at about 750 rpm. After about 15 minutes of creeping along (letting clutch out and revving engine to go and then stopping) the idle speed started creeping up. It got all the way to about 2,200 rpm before I just turned around and headed back to the house. There was no traffic in the return direction so after driving along for about half a mile at 45 mph, it settled back down and worked like normal again.
It's happened about 4 times since. It always goes away when getting back up to speed and normal driving. It's almost like the automatic choke is kicking in.