Apologies, this is the problem with play by post.
I'll do.my best;
You were at a crossroads, approaching from south.
West is the signals room, open door.
East is the toilets, closed door.
North goes to fire exit.
As you go north to fire exit, you cross path of signals room.
Staff hear your footsteps and call out to you, thinking you're the guy in the toilets. You hear the call through the open door.
Hope that helps mate, I'll draft up a diagram if not