If we're assuming external features utilized to improve scouting capability (e.g. Wizard cast Invisibility on the Rogue) then it's also worth noting the Imp can equally be the target of any such spells (though it obviously doesn't need help with invisibility).

I know we can't get a full picture without looking at all possibilities, but I would recommend we start at level 3 (for all potential scouts) and work from there whilst treating the Imp as equally buff-able.