1) Reflected light, if they can see something suddenly reflect at 25 metres they are not likely to hang about and wait to find out what it was.
2) Outline break up. The main reason people wear camo is to break up their outline, makes you harder to spot, therefore the gun is harder to spot.
3) Silhouette break up, with a loose overhanging shroud the outline of the gun is broken up even further.
I have some photos to illustrate what I mean.
Gun in field!
Gun in field with shroud.
Sonofafemaledog to find sometimes gun in the field!
just looking at the bottom photo