LE probably pinged her in the general vicinity, they then looked to see where a body might be hidden and the park was a big ol glaring neon sign blinking "LOOK HERE."
The problem with using cell towers is that they don't give you a precise location, it only puts you in a general area. Unless things have changed the basic process works like this: you have cell towers A, B & C. Lets say each tower covers a 3 mile radius (not all towers do this, some only have one antennae where the signal spreads out in a fan pattern, the tower needs an antennae on each side to provide radial coverage). So what happens next is you draw a 3 mile circle around Tower A, another around Tower B and one around Tower C, where the circles intersect each other is where the person's phone is located. That area could be as small as a city block or as large as a square mile or so. Now lets say Tower A has a single antennae that spreads out in a fan pattern, the entire area covered by the fan where it intersects with the signals from Towers A & B now becomes the search area and that area could easily be between 4 & 6 square miles or more.
