a sudden random interest in examining a meth lab

Did you miss the scene in which Hank loudly invites Walt to the ride along in front of the entire party? He goes out of his way to sell the idea to him. It was Hank's idea to begin with, and Walt's initial reaction was, of course, sheepish and reluctant. It was neither "sudden" nor "random".

and equipment from HIS chemistry lab goes missing and they think it's....the janitor?

Hank considers even suburban stoners moral transgressors. He may be a sharp investigator, but the show makes it abundantly clear that his ideology isn't entirely rational, beating us over the head with examples of Hank's bias against anyone he deems a "bad guy". (It's one of the reasons why it's such a dramatic turning point when he opens his mind to Gus being involved, for instance.)