In Breaking Bad S04E12 End Times, at the end Gus walks towards his car at the hospital parking with his two bodyguards, with Walt watching from an adjacent rooftop. As Gus is approaching the car he stops dead in his track with a doomed look on his face and heads over to the railing. He looks around but is not able to see Walt or sense anything out of place (as far as the viewers can tell). But he still walks away from the rigged car.
How does Gus sense that there's something wrong with the car? Moreover, Gus is never shown to be paranoid or doing things over a hunch, so how is this explainable?