Have you noticed that sometimes itoo's Forest Pack wants to create some "nodes" just before rendering? Not sure what causes this, but I've seen it now in a Max 2015 scene with a large Forest using Mental Ray Proxies. Max throws a warning upon rendering if the current ACTIVE LAYER is hidden. Make sure your current active layer is NOT HIDDEN and can be written to for these temporary Forest Pack nodes before submitting for render. These Forest Pack nodes are deleted as soon as the render is finished, however, they can create an extra layer too that is suddenly empty when the render is finished. Not sure what specific scene/forest pack/max settings are creating this odd occurrence or if this is something endemic to Forest Pack now. Anyway, it seems like a strange thing I came across and wanted to pass on. Happy Rendering!