According to the PH: "When a creature can’t see you, you have advantage on attack rolls against it."
"In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you."
Being hidden generally requires some sort of obscurement. Most players don't want to take the penalty for the attack, and this seems counter to the idea of "getting the drop" on the bad guys.
My quick and dirty house rule is that the character can take one step (5' square on the grid) out of cover and still get the benefits of being hidden, as long as that's all he does before the attack. This simulates the standard movie scene of stepping from behind a car/crate/building and popping off a shot or two, then ducking back in. The counter to this is for the target to ready an action to shoot the annoying putz.
As an additional rule, I go ahead and interpret the rules on needing to be unseen to hide to include having full cover. The rationale is that hiding represents being a bit tricky in how you time your next attack -- I'm ambidextrous with a gun (right hand dominant, left eye dominant balance out) and have pulled off the "which way is he coming?" many times, in paintball. I generally give the target advantage on the Perception check, though.
What about out of combat?
Attacks never, ever happen outside combat, by definition. Starting the attack could be what triggers the roll for initiative. Successfully hiding, deceiving, etc. could result in your opponent being surprised. If they aren't surprised, then, yes, they could actually win initiative and you could be up a creek. If you suck at ambushes, don't do them.
Situation 1: A rogue makes a successful stealth check and sneaks up behind a bored guard. I assume they are considered "unseen" and attack with advantage (and the guard is surprised).
Correct. Roll initiative. Even if the guard wins, he's still surprised. He may have picked up that something is amiss and is in the process of gathering his wits, but he hasn't done so. You're hidden until you come into view (probably the attack roll, but depends on exact terrain).
Situation 2: A character in a busy crowd walks up behind a bored guard and attacks (no stealth check). Is he considered unseen?
No. The guard can still see you. Being hidden is, quite literally, being hidden.
The gray area comes in whether some non-stealth deception was used to look innocuous. Personally, I allow Deception and even Slight of Hand to be used, in certain circumstances, to surprise someone. Some DMs take a more literal interpretation of the surprise rules and only allow Stealth vs. Perception, no exceptions. Depending on exactly how the player went about it, I might let him use Deception instead of Stealth to hide amid the crowd. Probably not, though.
Either way, you aren't hidden. The guard may just be surprised -- which is good for other reasons, especially for Assassins.
Situation 3: A Warlock with Mask of Many Faces makes himself look like a friendly NPC, walks up to a bored guard, passes behind him, and then attacks the guard from behind. Is the Warlock unseen?
IMO, this is just surprising, not hiding. What I'd probably prefer is for a successful Deception check (with whatever benefits the Mask may give) to allow the Warlock to try for Stealth at advantage, due to the guard casually turning his back on his "friend". In this case, the Stealth check still shows how well the Warlock got into a position from which he cannot be seen; everything else is just a complex skill challenge.
That's the theory, anyway. If no real benefits were awarded (i.e. no Sneak Attack from multi-classing), I'd probably short-hand it and say, "Use your Deception to hide." That tends to bite me in the butt when the Rogue player decides that he's going to ride that short-hand ruling.
