MT - just transmitting a Squawk code doesn't tell anybody the aircraft's position, only its identity and its altitude. RB puts those aircraft in the list, working out the aircraft type by matching the Mode S code in the databases.
RadarBox can't pick up position information from aircraft just fitted with Mode S because it's not being transmitted - only aircraft with ADS-B do that, so while they appear in the list, they can't appear on the map, because only "proper" ATC, with its radar, knows where they are! (actually, with any luck, the pilots do too! :-))
Rod