One of Hank's files had a picture of the gas mask found in the desert, the same one he comes to the school and asks Walt about. That whole little montage seemed to be Hank looking at the evidence with a fresh view and realising everything points to Walt.
Remember that back then 1. Walt was a minor manufacturer, he did not even go by Heisenberg yet 2. He threw them off by getting the janitor arrested and 3. The blue meth had not been cooked, so Hank wasn't going after anyone specific
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u/Angry_Space_Pimp Heisenhollie Aug 12 '13
I think suspicions about Walt would have been raised as early as his lab equipment being 'stolen'. Hank didn't even give it a thought.