Warnings were not necessary.  Snipers were omnipresent and by March 1915 trying to avoid them was common practice among British troops.  Other material I've read indicate that George thought he was safe and did not know he was exposed.  Death by sniper was common.  George's friend from Eton and Trinity, Chris Lawrence (grandson of Baron Lawrence, photos of he and George posted recently by Andrew) died by sniper fire 13 October 1914, only days after arriving at the Western Front.