ok, another way to find it is to load the obj to uvmapper , and use color by material or select by material , specify default and have a look at what pops up.

Its possible I suppose that the mtl file got a material written to it that has 0 polys assigned. Lightwave will assign a material with no polys, but it doesnt write it out to the file.

 

I have attached a script that will list all the usemtl lines in an obj but you need to edit the filename to point to your file.Rename from txt to py.