Those guys were transferring the box into the armored transport.
Couslon and Mack discover where the exchange of the weapon, a mysterious box, is set to take place and stake out the facility. When Chen and his goons begin going nuts, shooting each other, Agent May sails in to take them out.
This was a fairly difficult location to track down. It was a warehouse called Norris Industries, and luckily, the floor pattern of the interior helped give it away. This was the same location that appeared in the very first episode of Agents of SHIELD, and was used for the factory where Mike Peterson worked. The word Norris can be seen on some of the green machinery during the shootout.
MAP
(Agents of SHIELD S04E01, 2016)







