Which valley are you talking about? If you're talking about Peters Creek Valley (trailhead top of Ski Road, I think) or the trail from the Old Glenn up to Mt. Baldy, those are closed to motorized vehicles, always. If you heard snowmobiles, you either heard residents using their sleds on their land, or you heard people who weren't following the non-motorized rules in that area and are risking fines. There aren't a lot of places you can (legally) ride in the Eagle River/Chugiak area.