Interesting idea, and makes a great deal of sense to me.
I've been thinking about his driving route, his returns to the crime scene, and the mobile phone issue. First, it seems clear that BK assumed that LE would only check cell towers near the crime scene, and he felt comfortable carrying his phone along and only turning it off when he wanted to hide his whereabouts in a specific locale. As to why he carried the phone in the first place, perhaps he wanted it to keep from getting lost on his return trip. He may not yet have felt familiar enough with the route or may have just been the kind of person who relies on cellphone GPS all the time.
It appears that he didn't turn off the cellphone during the 9:00 a.m. (-ish) trip back to the area of the house the next morning. I don't think he had any idea of going back in to get the knife sheath. He would be very vulnerable in daylight, and in any event, by that time of day, there would be people up and about in the neighborhood. A simpler explanation is that he just went back to see if anything was happening.
What's interesting to me is that he turned off his phone for a period of time the second evening when he was in the area of Johnson, ID. Apparently, he wanted to be sure his whereabouts at that time couldn't be traced. If it was indeed Johnson, ID, I would assume that he was getting rid of incriminating evidence in some remote place there...but if it was Johnson, WA, and near his return route of the night before, it could be that he had already ditched/buried those items the night before (makes the most sense to me), and then went back the second day to get rid of more items (towels and such) from his post-murder clean-up or to check after suddenly realizing that he couldn't remember anything about the knife sheath.
I wonder if the knife isn't buried somewhere around there, or if there are any bodies of water near the road in that area.