A senior Home Office official has told MPs the department does not know the whereabouts of more than 17,000 asylum seekers whose claims have been withdrawn.Permanent secretary Sir Matthew Rycroft and his interim second-in-command Simon Ridley faced questions from the Commons Home Affairs Committee about the backlog of asylum claims waiting to be considered by the department amid a race to meet the target set by Rishi Sunak to clear some older cases by the end of the year.