I think he's just watching the hostage's faces for reactions or using a polygraph to detect changes in their or Gakuto's body that would correspond with getting nervous because he's close to where his ships are. No way plants are talking.
Edit: So I went back and checked the chairs Kaji and that other dude are sitting on, and there's straps on their wrists, neck, and chest. I think there could easily be electrodes hidden in those, and what Voja brought is actually a device that receives their polygraph readings in real time, allowing him to read information from the hostages in real time, since they're also watching through the camera, it seems. This makes sense also because Voja was in the corner looking at them while they were tied up, and Voja brought Captain Lacy that device that he's using to "read the plants". So I'm pretty sure that it's something to do with that. Certainly seems more plausible than whatever the Captain wants Gakuto to think.