When adventuring it is very important to stay away from grues. I have developed a grue-detecting t-shirt which will warn your party about potential grue danger. When in a good light source or using your lamp, the t-shirt will look as it does in this picture.
In low light conditions the t-shirt looks like this. Find a light source quickly.
I had this prototype made at spreadshirt.co.uk; in case it's not obvious, it's a combination of reflective and glow-in-the-dark vinyl print. Unfortunately, spreadshirt can't guarantee the alignment of these two print materials, otherwise I'd be offering a link to buy these essential pieces of safety equipment.