What do you do when the silence after 'I'm fine' feels heavier than the lie?

I used to think my job was about preventing collisions. Now I wonder if it was just learning to listen for the ones that never got said. That moment when someone says 'I'm fine' and the air between us thickens—like a runway with no traffic but all the lights still on. What do you do with that kind of quiet? Not fix it. Just sit with it. Or maybe walk away. But not before noticing how heavy it is.