Tazren, actually tons of people have seen and know Taim. He was one of the most famous people in all of Saldea by proclaiming himself as the dragon reborn. He had followers, an army, which apparently had some run-ins with Davram Bashere. I wouldn't say that Taim was especially secretive. If Ishamael is impersonating Taim (as you think) I believe that he simply could not risk running into someone who knows Taim, so he'd have to impersonate him pretty much exactly.