"Home Office officials have admitted they do not know the whereabouts of more than 17,000 asylum seekers whose claims have been discontinued.
Officials told the committee that claims are withdrawn if asylum seekers fail to respond to two successive case worker interview requests or questionnaires.
When an application is withdrawn it means it is no longer being considered and if the claimant remains in the UK they are here illegally and can be removed."
https://www.bbc.co.uk/news/uk-67567401