If they are running up debts then most likely they have done a runner. Change the locks once you are sure. With reagards to bailiffs. Contact any that leave notice or call, and explain the named was a lodger, that they left no personal goods of value. The bailiff should then refer back to their client whom I'd advise you also contact directly in writing, and explain the situation. The bailiffs shouldn't be an issue (as they have no right to force entry as long as all windows and doors are closed) but you will need to check that the debtors haven't listed your address with a credit agency.