Let’s say you’ve got the whole party together — we’re not talking an independent scout here necessarily, but just the party going from place to place.
If two of these groups collide, who surprises whom?
Encounter distance. Just like OD&D.
The party has an encounter distance within which they spot anyone not specifically hiding from them. Ditto monsters. The party with the broader encounter distance has surprise at the moment the encounter begins, and it begins at the moment the radii overlap.
If someone is specifically hiding, clearly they’re not surprised (they know that the other party is there!) and so play it out with stealth and perception.
Lanterns expand the size of the party to the edge of the light radius higher than the surroundings — effectively adding to the other party’s radius.
You could probably do the same thing with armor and encumbrance, projecting a sound radius that travels in front of the bearer, effectively adding to the foe’s vision radius.
Moving more quickly halves effective vision radius, more slowly doubles it.
Walls block radius; doors count as 20′ worth, etc.