With the new information; Bow missing, bullet holes in swapped tires and Bob's hubcaps, and Randy's mom calling Bob's family:
1) I'm now wondering if Bob had clothing and other items which were missing.
2) The Bow could have been taken along with the tires and battery.
Consider for a moment just who would take tires off of a vehicle, then would ditch their own but along with Bob's Hub caps, when the keys were in the car!? Why not just take the whole car? Again this points to someone local or tied to the local area.
3) What kind of bullets were in the tires and hub caps? Hunting rifle bullets, or hand gun bullets? All from the same gun, or different ones?(a clue that this may have been more than one person)
4) The car may have been driven to the location it was found, so that the tires and battery could be removed. And of the license plate, it too could have been taken for a lot of reasons. It does strike me as odd that only certain things (like tires, which take quite a bit of time to remove) were taken, but glove box was left untouched. This is someone thinking more along practical lines and not all out thievery. Why ditch the hub caps? They could later be an identifier, or could have been seen as flashy.
Working with the possibility of Native Americans, or some other locals the hub caps could raise eyebrows for multiple reasons. There are 2 reasons Native Americans may figure into this. There was a lot of tension with Native Americans vs. Hunters in that time period. There is also that more practical tendency to "Take what I need" and leave the rest behind. That kind of thinking could also be anyone, but it leans in that direction.
5) It is not unsual for deer hunters to drive around "spotting" although with Randy's mom's call it would appear Bob would not do something so erratic. It also seems Bob knew the area very well. So, now my view is shifting more towards an intersection with another person or persons. This place where the car was found, seems like the car was driven here so that the tires / battery could be removed without being noticed. Someone knew this location. Points to locals, or frequent visitor to the area.
6) What are the odds that 2 people from the same family would be murdered(as it now looks for Bob) a few years apart? This is statistically significant, and if nothing else, points to something going on in the family environment. Both Bob and his sister had exceedingly bad judgement and become victims? There is more to the associations involved with these people than a clean cut graduation photo. While there may be no direct tie with the bikers, there is some other thread here that caused these 2 people from the same family to be involved in risky circumstances. There has to be more info to get a Picture of Bob and what he might of have done for example: a) what if he was confronted by a group of people, on foot or in a car? b) How likely would he have been to pick up hitchhikers? c) what is the possibility that he was more interested in pot or drugs than deer? d) how reliable was he at showing up on time, etc...?
7) Exactly why was his sister murdered? We know it was bikers, but was a reason given? Earlier, think someone said she was an informant. This strikes me as strange. Girl hangs around with dangerous bikers and then squeals on them. That's playing with fire. If Bob's sister had those kind of balls, one wonders what Bob was capable of. Or was she having no fear of death because of her missing brother? Psychologists, help me out here.