You are referring the exact thing that won't stop troubling me. To me, there always has to be an answer to the question of why? Jurors don't have to know it to convict, but my mind wants to tie that up. Why did he go back?
This is at least a zebra of a theory (but not a unicorn): He went back because he had a meet-up precondition to the abduction. If he did not know the meet-up point when he left with her, he might have been instructed later. The only logical way is by phone, unless he was being followed. Christina could have been transferred to another person's vehicle for purposes we can only surmise. However, this whole theory is nothing but surmising and there some really good sleuths here (as I am not). I am theorizing that someone other than EA took her body to another destination. So go ahead all others and poke holes in this theory, because that is what I think we ought to do and I won't be at all offended.
If, going by what the police think, that EA left the garage with CM in his trunk, having spoken about the time frame being 3 minutes, perhaps he went back to make sure there was no evidence left. Nothing fallen out her bag, no blood, no police presence?
Perhaps CM was still alive at this point, maybe she got in EA's car volunterally, did he say he would drop her off? But something happened then?
If it really is 90 days they have to charge him, and time is approaching for them to do so. I would think IF there really was any other arrests to be made they would be done so by now? Would they not be on trail with EA? Maybe I'm wrong. But I don't think there will be any other people involved now.