If he has to guess, it probably responds to the vibrations caused by sound and movement. Up in the air, Zidane has not disturbed anything but the wind.

The vines wriggle towards them despite the whispering and he backs them away a little further before responding.

"I will act as bait whilst you fly up and find its core. I have a jar of saliva from that 'pelican' some moons ago which should destroy it - or at least weakening so we can finish it off."