Deliberate mistake: In the scene near the beginning of the movie where Ben finds the pipe and finds the message by rolling the pipe on the paper, the message is long and has symbols, but it is impossible for all of that to fit in the pipe. (00:12:00)
Deliberate mistake: In the two scenes when Ben and Riley are in the documents room, it is in full light. The rotunda is actually dimly lit as a way of protecting the documents from light damage.
Deliberate mistake: When Riley uses the video camera laser to disable the Declaration of Independence sensor, that could never happen since cameras are not permitted in the documents room.
Answer: He doesn't simply assume "Beneath Parkington Lane" means beneath the church: Parkington Lane is the name etched on the tomb hiding the entrance to the tunnels. When he saw it, he naturally deduced what he had to do.
Sereenie