We had a location like that, where the person that was supposed to let us in, accidently came out to get us from a smoke break, not bringing his card with him.
They showed us a neat trick, the automatic door sensor on the inside could be tricked, by moving a shadow along the floor making it think something was there and we could get in.
It was a very poorly made comparison camera sensor. It would take pictures, and when they differed enough it opened the door. They noticed this when the sun would move to a certain position and a truck would drive past.