“Island County detectives are looking for a knife that could be a key piece of evidence in a serious felony case.Detective Robert Clark is asking anyone who has found a knife on a beach in Oak Harbor over the last couple of weeks to contact him at the Island County Sheriff’s Department.He can be reached at 678-4422.Clark said the missing knife relates to an on-going investigation, but he wouldn’t comment on the specifics of the case.“It may be a very important piece of evidence,” he said. “