Circumstances will determine the details, but as a rule of thumb, I'd say 50-100 feet underground or 100-500 feet overland.
Something that works really well underground is to have the party wait at a point, have the scout move to the next point of interest, then return back to the party. Repeat as needed, and the party's noise/light/ect. shouldn't be an issue. If something bad might happen to the scout, they run back the party ASAP. This actually shouldn't take away from the game either, because while the scout technically "has the spotlight" longer, the DM only has to describe everything once, and the party will make decisions as a whole (rather than the scout deciding for everyone).
Something that works really well underground is to have the party wait at a point, have the scout move to the next point of interest, then return back to the party. Repeat as needed, and the party's noise/light/ect. shouldn't be an issue. If something bad might happen to the scout, they run back the party ASAP. This actually shouldn't take away from the game either, because while the scout technically "has the spotlight" longer, the DM only has to describe everything once, and the party will make decisions as a whole (rather than the scout deciding for everyone).