The head of the Swiss customs service has suggested Ireland could be seperated by an invisible border after Brexit.Dr Christian Bock, giving evidence to Westminster's Northern Ireland Affairs Committee, said a number of conditions would need to be met, including common border patrols involving UK and Irish staff.Dr Bock said there would still need to be customs "control points" but those could be at locations away from the border, such as at business premises.He said German police helicopters fly in Switzerland with Swiss customs staff on board.Dr Bock said there would also have to be an intelligence strategy, a pre-qualification system for trusted traders and a system for easing low-risk trade.