A candidate will apply for a specific job and any comments, scorecards or ratings that are noted against the candidate will be specific and relevant to that job. Therefore in order to find a candidate you would need to locate their application within a job that they applied for.
Additionally if the candidate was added to your Talent Pipeline, you will find them in there and there is a search box available. However you cannot view any comments, scorecards or ratings within the Talent Pipeline. Again, you would need to view their application within a job to see this.