I have seen posts where people thought that any strangers around Keddie would be noticeable, but I'm not so sure about that. Back then, there were more businesses like bars, restaurants, and stores open in the Feather River Canyon, and they catered to the tourist trade. Keddie was a resort town with at least the store, bar, and lodge, and I would think that there were enought tourists coming through that people would have been used to strangers around, and they wouldn't really stick out in anyones mind.
As for why no one heard anything, I can think of two reasons, besides the fact that having a T.V. on could drown out noise in peoples cabins. 1. Sue didn't scream because she wanted to keep the children safe. 2. Someone on the cabin 28 site mentioned how loud the trains coming through were, and one of them could have been passing through at the time.