A group of 4 PCs roll Dexterity (Stealth) checks. They sneak around and 3 of them get noticed by the enemy (because of line of sight or just bad Dexterity (Stealth) rolls). Combat starts, everyone rolls initiative.
If the remaining PC has a Dexterity (Stealth) roll higher than the enemy's passive Perception, does he remain hidden? Can he then go around to get an attack with advantage on that enemy (while remaining out of sight)?