I’ve only seen 2-3 weasels in my life and none were ermines in their winter white. I think they’re basically invisible then, although I do see their tracks. Even after decades in Alaska I’ve only seen a handful of mustelids.
I’ve seen weasels quite a lot, especially during the winter. They don’t seem as afraid of people as most animals. I actually saw one carrying a dead mouse just about a week ago.
Not incredibly often, no, but there are lots of burrows along some of the hiking trails in Point Reyes Nat’l Seashore (which I visit fairly frequently). Every once in a while, I’ll catch a glimpse of one poking its head out of a hole, or darting across the trail. They move so quickly, though, I’ve never managed to get a picture of one.