I'm guessing he was careful about getting rid of his internet history, but yeah I agree he wasn't so careful to avoid creating cell site location data that might connect him, not to mention the suspect gap at the time of the crime. He could control his internet history to some extent after creating it imo, not so much his phone pings.
When I try to imagine BK's mind set, I've wondered if on ...X... number of occasions if/when casing house prior to the murders, he was (in his head) "just watching" and fantasising; so the phone stayed on and pinged away. Commitment to go further was not yet formed/was still fantasy. All my own speculation.
But on the night of the crime I think BK had already formed an intent to enter the house and kill whilst still in Pullman. He turned the phone off (not aeroplane mode imo) and took it with him (I have one theory for why I think he took his phone but not for this post).
It will be interesting to see what the P has in terms of pre-crime BK CSL data come trial. I actually just read State's objection to Change of Venue for the the first time ( linked below), and it seems clearer to me now the state definitely does categorise BK's pre crime CSL data as potential trial evidence (this doc shows clearly state distinguishes this CSL data from stalking rumours imo). If interested, relevant part is Section iii pp 11-12 re the court record, admissible and inadmissible evidence.