Total guess, but to me it looks like an AO map overlaid with a generic dirt/grime image (e.g. from CGTextures). So you would bake out the AO, set up the nodes to mix in the dirt/grime image and then bake out again.
Once again, that's just a guess. I am sure Kent will be along shortly to answer definitively.
Kent Trammell
replied
lewis2e is correct: It's simply an AO bake with a generic grime texture on top set to "Overlay" blending mode.
I go over this kind of grime in depth in the Sci-Fi Helmet shading course: https://cgcookie.com/course/shading-the-sci-fi-helmet/
