Funny, you should ask because my husband and I were having a similar discussion last night. He was especially bugged that Deatheater!Snape had no knowledge of Peter's interactions with Voldemort, if indeed he was feeding Voldie information for over a year. I suppose the only people who know about this were Peter and Voldemort -- the better to keep this a secret.

As was said up-thread, Dumbledore seems to miss a lot. He also believed in Sirius's guilt before Peter's deception was revealed, didn't notice Imposter!Moody, Scabbers was Peter and that MWPP were animagi.

Peter probably was rather quiet and thus underestimated.