I dare say that the director did not actively use a scientist to formulate this trick. I base this on the presence of goofs in the script that a scientist would not have made. However, it is certainly scientific and possible. I expect that the scriptwriters pieced this together based on three things:
- Triangulation: This is a pretty common trope used to determine the position of a transmitter, often a mobile phone. However, this is not what Neeson actually uses.
- Lightning strikes: There is a rule of thumb which can be used to calculate the distance of lightning strikes from your position based on the delay in the sound of thunder. Neeson's use of grenades is pretty similar although we have to presume that cellphone signals are instantaneous.
Trilateration is the slightly more sophisticated cousin of triangulation and involves intersecting circles (or spheres). It is how GPS works. This is what Neeson uses in combination with the lightning strike trick to approximate distances.
While the image above show three circles, two are sufficient to get a rough approximation.
For the purposes of a script in an action film, the technique holds water. As to whether it is possible for "real people" to use such techniques, I'd say yes. But it's not a trivial calculation. Given Neeson's military background and the technique's potential for use in the field, it is possible that he would be aware a familiar with it.
How Stuff Works has a page which explains 2-D trilateration in easily digestible form.