The log specifically states that they attempted to "ping" her phone. That is the cellular method of establishing a connection between the cell on the tower and the physical phone, initiated by the tower cell, which inherently locates the phone's exact distance from the tower. That distance can be easily plotted on a map as an arc across the cell coverage area. In this instance, there were few pathways available to her that would have intersected that arc, giving LE an excellent and quick idea of where to look. Since the ping did not locate the phone, which they knew was most likely within the cell coverage, the logical conclusion was that the phone was turned off, or disabled.
I have placed a red arrow on this screenshot from Cellmappers.com showing the narrow cell that her phone would have been in. IMO
Verizon (United States of America) Cell Tower Map