BBM
I tend to think also that Whitney was assaulted (my guess is sexually, after he pushed the back seats down),
or was at least very frightened by JH, at Dodge Park.
JH probably got out of the car for some reason (to urinate?) and had the car keys with him.
Whitney then locked all the doors, but JH smashed the window.... and then killed her.
If this happened then there would be glass inside the car and probably some on the ground at Dodge Park.
He then drove to Larch Mt, where he left her body.
Opening and closing the car doors would cause some of the glass to fall onto the ground there.
I think he might have removed the floor mats and shook them (some where) to get rid of some
of the glass inside the SUV and then just tossed them onto the back seats (that had been pushed down).
Or he may have gone somewhere to vacuum the mats (however it doesn't seem as though he had much time to do so.)