Roof access door- can it be locked from outside or against code?

We have a full bulkhead with standard stairs to the rooftop. It is a standard full-size metal door which is always unlocked, both from the inside and outside. I realize it obviously needs to be unlocked from the inside for egress, but does it need to be unlocked from the outside? It just would allow anyone from a neighboring building to come into our building via the rooftop. Is there a code that covers this? Does it need to be open so firefighters can get in if needed to access from another building? Any thoughts?