No, it appeared when loading the tgd file together with an error message that an mtl file couldn't be found for an object. This wasn't really a problem though as I had changed an image map from the original and not updated the mtl in the previous session.
Another odd thing that may or not be related was that an object that was in the scene was not listed in the "select object or shader" option when right clicking in the 3d view window. I eventually got it to be listed by changing its name slightly. It then reappeared as normal.
