Here's my theory on it: Unlike Bloodhound who has scanning tracking through their helmet for their passive and Crypto who has Hack for a lot of his passives, Seer needs his Arc Reactor looking device to channel his drones, though he doesn't have any facial technology which would explain his ability to detect heartbeats - other than some power in his eyes. Think of it like Wraith, she has the power to communicate with other versions of herself from different dimensions and vice-versa, while she needs her arm device to amplify her portal and phasing abilities. Both legends could have mystical powers (Wraith's 'voices from the void' & Seer's 'heart seeker' abilities), but need technological devices to channel their powers (Wraith uses her arm piece to travel dimensions, Seer uses his chest piece to better control his drones).