Grots don't loot anything from the living. No need to sneak upon live eldar to get junk of their most common weapon. He could loot the battlefield, steal from some artifact trader or just trade for anything. That's just the basic ways. If there be need I can imagine a dosen fluff-valid funny or dramatic strories, you just have to think a bit out of table-scale to do this.
Man, you can't be seriuos? If the battle is lost and the looters roam the field who can remove the dead? /facepalm Not saying that there a lots of factions within eldar society like exodites, corsairs, rangers and who now how many individual eldar scattered across the galaxy. Yes, definitely none of them can't loose/sell/dispose etc of petty broken shuriken pistol... Lets stop this useless conversation as your point doesn't seem to impress me.