May have just been a colour density test patch. Are there any decals on the sheet that incorporate silver?
Hobbycraft is known for puting "extras" on their decal sheets. I built one of their 1/48 DeHavilland Vampires and there were at least four extra decals on the sheet that no reference was made for in the marking instructions.
When in doubt, leave it out. I wouldn't worry about it or use it if it isn't refered to in the marking and painting guide.