If we was confident enough (falsely, it seems...) to not be identified by LE because he was not associated with the victims, he might have presumed that one approach would be to look for phones pinging in the area at the time of the murders. If his phone was off, he would be excluded from that report. For example, it's definitely possible that LE checked cell phone tower data to verify alibis. If he felt confident about not being on LE's radar, at least as part of his plan before committing the crime, it might not have mattered to him that previous cell phone data linked him to the area. JMO -- OP, you're absolutely right!