It would be informative to see the official report, which ought to give specifics and answer some of the questions raised here.. Just a couple of comments - a signal mirror will work, although with diminished range, in overcast conditions. Just how thick were the clouds?

At night, or in bad weather, a fire is one of the best signals. The light, if spotted, is quite distinctive, and the smoke, especially, will bring searchers to you. I have followed smoke upstream on a couple of occasions during damp searches.