The squad lives or dies on the read it takes of a compartment before the door blows. Scanning & Detection covers what your senses, your sensors, and your specialists bring to that read; and what happens when one of you slips ahead to do the marking solo.
In order to be considered Scouting, any creature you come across has to be unable to detect the group you are scouting for. This could be a single isolated sound-proof room away, or 5 clicks on an open plain. Essentially your group has to be out of the perception range of your target.
When this situation arises you are Scouting, and Scouts go first in Combat Order.
If you also avoid the perception of your target, then your target is Off Guard. How much Off Guard is determined by the skill difference.