My great grandfather was an American army medic in WW1. He told me once that he became buddies with a British soldier who had been fighting since the beginning of the war.
Apparently the British guy told my great grandfather that once during the battle of the Somme (a three month battle in 1916 that resulted in over 1,000,000 casualties) during an unusually quiet night him and his squad mates had been keeping watch in the trenches when they saw something moving out in the no mans land. It was something tall, thin, and completely shrouded in dark material. He said it seemed to glide across the ground, and ever so often it would stop, and kneel (he was pretty sure it must have been stopping at dead soldiers). After only a minute or two he said it seemed to fade into the darkness like it had never existed. He, and the rest of the men around him were all to terrified to call out or take a shot at it.