Hank removed the spare car key or key-hider from Walt’s car when he checked it. In the scene people usually mean from Breaking Bad season 2, episode 2, he goes to the back of the car and takes the hidden key so he can inspect or use the vehicle.

What he was doing

He was looking for a way to get into the car and possibly follow a lead connected to Walt, not stealing the car itself.

Why it matters

That moment is part of Hank’s early suspicion and the show’s habit of hiding important clues in plain sight. It also foreshadows how closely Hank ends up watching Walt later on.

TL;DR

Hank took the hidden spare key from Walt’s car, then used that access to check it more closely.