IMO, we now know that SM was having coffee with the neighbor when she sent the "I'm done" text. I believe it's fair to assume she may have "spilled a little tea" with the neighbor at the same time regarding her plans to leave the marriage, her fears, etc. If so, the neighbor would have immediately communicated that to LE when they arrived on site at the PP home on the 10th.
And that's all it would take to put BM in the crosshairs from the outset. And it would explain why they prevented him from leaving the scene when he tried to walk away. And why they seized his truck immediately.
MOO.