Why does Gus walk away from his car?

From this source:

"Walt was right when he said that Gus is always 10 moves ahead. Gus didn't need to be right about his car being sabotaged; he was just smart enough to know that he was walking into what would be a perfect trap, and one he’d happily spring if the tables were turned. So he walked away."

Basically Gus wasn't sure if his car had been rigged to blow, but he knew if he was trying to kill someone it would have been a perfect way of achieving this.

He is clearly very good at reading situations and this lead to him being so successful in such a dangerous industry.

As we know, Gus is an extremely careful and intelligent man and he is in fact always ahead of Walt and everyone else. And I think it was exactly the fact that Brock was poisoned (which we know Gus didn't do) that made him doubt the situation, which is told to him by Jesse during their meeting right before.

Jesse: He's not sick, he was poisoned.
Gus: How did that happen?
Jesse: The doctors, they don't know.

So he learns that Brock was poisoned by unknown circumstances. Gus might even guess it was Walt or not or that Jesse is in on anything or not, but what he knows is that definitely some kind of foul play was at the works and ultimately ended up bringing him here. So I think this was the primary hint that something was wrong with this whole hospital visit. And in fact after he is told of the poisoning he immediately gives in, lets Jesse stay in the hospital and leaves, though he doesn't seem to think about an actual car bomb until on his way to it.

The entire situation (being lured to the hospital by Jesse,leaving his vehicle unguarded,etc) seemed rather odd to Fring. While he did make a serious tactical mistake (he walked to the edge of the parking structure and looked over, thus potentially exposing himself to a sniper) he seems to have gotten a bad feeling about things and decided not to drive away in his car.

Upon reflection, this actually is more a affectation of the narrative than a practical exercise as later (SPOILER) when Walt and Tio Salamanca use the same explosive device to finally murder Fring, it is clear that it’s explosive effect of the bomb would have been sufficient to seriously injure or kill Fring had he walked as close to his vehicle as he did. There was only a limited need (primarily for the sake of the series) for him to actually get into his car.

Walt used the walkie talkie to detonate the bomb. When he turned it on Gus could hear the background noise's and Walt talking to himself.