I'm not sure I understand the layout, so if I have it wrong forgive me.

If there is a full door between the folks staying late and the guys leaving the warehouse door open, it needs a lock controlled by the folks staying late. If there is such a door, the lock should be an easy installation.

If there is no door, can one and a lock be installed at reasonable cost?

Prevention is the best cure for a situation like this, and a simple door with a lock and "buzz to enter" system may an answer.