I missed it the first time I watched the series but on a second watching I just noticed after Walt goes missing (taken by Tuco), Hank goes to Walt's house with the APD detective to ask Skyler some questions.
Just before he goes inside the house he feels around the car and grabs a magnetic device from underneath the right wheel arch. This never gets mentioned again.
Considering this is so early in the series it raises a lot of questions for me:
- Who put the tracker there? It probably wasn't Tuco because he used Jesse to lead him to Walt.
- If it was Hank, why? Did he suspect Walt so early on?
We know Hank likes putting GPS trackers on people he suspects but the way Hank feels around the car suggests he doesn't know where exactly it's placed, suggesting it wasn't him that put it there (unless he just forgot).
Could it be that Hank had Walt sussed from the start and discovering Gale's book with the WW dedication just confirmed what he always suspected?
Here's the thing he retrieves from the car: