I still go back and forth on this, and I think I would have to rule on a case-by-case basis based on the situation.
Thats the intent in the rules though; no-one is arguing otherwise ('the DM determines when circumstances are OK for hiding').
My position is this.
N (creature without Skulker or similar ability) is in darkness.
C is also present (has darkvision 60').
N is 50' away from
C, and within the range of
C's darkvision, and thus
N is treated as if they were in dim light.
1)
N cannot hide from
C unless the DM rules
C is sufficiently distracted (i.e. looking the other way, and thus effectively blinded with respect to
N).
N can try to Hide (via the Hide action if actions matter, by standing there for a few seconds totally quiet while
C stares at him), but all such attempts automatically fail (for obvious reasons).
2)
N can move outside of the range of
Cs darkvision, and into the darkness (total obscurement) 10' behind him, and attempt to Hide once heavily obscured (again using the Hide action if actions matter).
3) If
N becomes hidden in the Darkness, and then re-enters
Cs Darkvision range of 60' and approaches
C,
N is
automatically discovered by
C, because
C is assumed to be observing the area sufficiently enough to spot
N as soon as
N emerges from the darkness into an area he cannot hide in (dim light), unless the DM rules
C is 'sufficiently distracted' and looking the other way.
4) If the DM rules
C is looking the other way, but
C is otherwise alert and listening for hidden enemies (i.e. searching) and looking around (but not behind him) he can make a Perception check to detect
N as
N moves up behind him (effectively,
C is taking the Search action).
5) If the DM rules
C is looking the other way, and
C is also not alert (he's busy reading a map or taking other actions), he is not taking the Search action, and unless
N does something to reveal himself, or the situation otherwise changes,
N can sneak up behind
C and stab
C in the back.
Note that the general rule is that
N is automatically discovered as soon as he enters the range of C's darkvision. The circumstances might dictate otherwise, and call for a DM ruling (C's player might have told the DM he is expressly looking the other way to the direction from which N is approaching him or similar), but the general rule of automatic detection holds in most cases.