I can see how he may look stupid, but this is where I might actually think it was ok. Simply based on he and Yoda's conversation later. He didn't think it was erased from the archives because he believed it to be impossible. And it may not have been so much that Obi-wan didn't think of it, but that Yoda wanted to give one of younglings a stab at real jedi investigation work. But Obi-wan to me doesn't seriously consider the erasing of the archives until Yoda explains to him that only a Jedi could have done that. Maybe that information wasn't available to Obi-wan yet... Anyway, maybe a stretch, but I can rationalize it. Wish I could the rest of the movie!