The Police Tactical Unit of New Territories South (PTU NTS) and Lantau South Division (LTSDIV) conducted a rural sweeping exercise, codenamed "ASHWELLER", in Shek Pik on Lantau Island on December 5, 2017, to enhance officers' capabilities in responding to emergency incidents.

The exercise simulated an escape of some prisoners from Shek Pik Prison to the surrounding hills, and some suspected illegal immigrants (IIs) were reportedly hiding in the vicinity. PTU NTS rushed to the scene by flight with the aid of the Government Flying Service, and worked with LTSDIV to tackle the incident. Observation posts and backstops were set up under co-ordination of the command post. PTU officers went uphill through multiple routes to conduct a thorough sweeping in a bid to outflank the escaped prisoners and IIs. Eventually all offenders were located.