I have another theory: they are cordoning off the block of flats and police officers remain there during the night (according to
Daily Mail), so they must have found some sort of (strong) evidence. Maybe they have to examine that evidence and give more information to the press in the morning. The press still has headlines like „the search is still ongoing“. So if any suspicious person is trying to leave the flats during the night (thinking nothing has been found) police will be there to capture them. Just my 2 cents. [Sorry, English is not my mother tongue.]