Fact is ... authorities say they may never know precisely what caused the fatal contact. The reason they're sticking to the headboard theory -- the Medical Examiner concluded Bob hit his head on "something hard, covered by something soft."
It looks like Bob used the bathroom before getting into bed. There's a towel in the tub and toiletries in the shower as well as on the counter.
What's interesting ... Bob may have taken a shower, although authorities haven't said that. It leaves open the possibility Bob may have suffered his injury in the bathroom, although it doesn't align with the theory of something hard covered by something soft.
There's also the interview cops did with the hotel valet, who says Bob was in good spirits when he arrived just after 2 AM. The employee said Bob showed no signs of dizziness, slurred speech or other indicators of a head injury ... and there was no damage to his vehicle indicating he'd been in a wreck. Further supporting the theory the accident happened in his room. He also looked fine in a photo taken with the valet.
TMZ broke the story ... Bob's body was discovered hours after he checked into the hotel room. Once he shut the door, no one else walked in until security entered the room in the afternoon.