Ethereal creatures attacking from the ethereal plane with surprise?
DM: As you walk down the hallway, you get the sense something is not... right. A shimmering and crackling energy sizzles in the air around you, almost a disturbance in reality itself, and you feel as if you can hear distant gibbering voices, almost as if it was coming from another place entirely. You're all surprised. Roll initiative.
Creatures plane shifting in to attack the PCs with surprise?
DM: As you walk down the hallway, a sudden horrible screeching can be heard, as a tear in reality itself opens in front of you. Spewing forth from this hole in reality you see creatures appear snarling at you with weapons drawn. You're all surprised. Roll initiative.
PCs ambushed by an invisible stalker?
DM: As you walk down the hallway, you notice the air swishing around you in an unusual way, and hear an unearthly howling as -something unseen- launches itself at you with ferocity. You're all surprised. Roll initiative.
Or whatever. It's s simple question of narration of the shift from narrative time, to combat time.