mach1.9pants
Hero
If it is one enemy yes, but he can only have the ally in between him and one enemy. He can't be behind for two enemies (apart from the 5' doorway or corridor situation), so to me, fine.
There's certainly some DM fiat involved, but I would say if you're in a significantly different spot on the battlefield than before, and have stayed unseen getting there, hiding wouldn't be a problem.
That's the difference between 5e and earlier editions. It's not condition a), then b), then c), then d), then e), then f), okay, now your hidden. It's here's some simple rules, now use some common sense to extrapolate.
But if you go this route, when is he tracked down? It would have to be never, or at least not as long as he has an ally near him. Every round he attacks and the enemies can see him only until he mysteriously disappears again until he attacks next round.
I bolded the part I'm trying to get at with my example. I think different DMs are using different definitions of "significantly different".
Is "significant" a whole move away? 2? 10'?
If you don't allow the halfling to do it whenever he can, that means it only works when he is basically following the meat shield around all the time, a one shot go at the start of combat as long as it starts with him behind his ally and in between a target. I like the idea of being a PC literally tagging behind the fighter, but often the Rogues get sent ahead to scout![]()