My speculation: I think that BL went off hiking to "cool off" during an argument and left GP at Spread Creek by herself, possibly without keys and/or her phone (or without enough phone service to get a hold of her friend, whom she was supposed to contact to meet up with that day).
BL knew that GP would be upset about him deserting her and missing her friend's birthday and, after Moab (and Merry Piglets?) he was sick of other people getting involved in or "putting their nose into" their arguments, so he didn't want the driver around to witness whatever Gabby's reaction was upon his return.
Ah, very interesting....IMO, he would have taken her phone. Can't remember when the odd texts were sent... previously, I assumed that he sent those and she was already dead, but maybe not?