Can an Investigator move directly to the balcony without using the stairs in the Foyer?
No. There's no special case. The division between those areas is marked with a white border. The movement cost to cross a white border is 1. 

I believe the FAQ addresses this issue specifically and says that it represents characters climbing over the banisters.

EDIT: I realized that if you read just the title of your question, it seems like I'm saying the opposite of what I really am. So:

YES - you can move directly to the balcony. NO - there is no special case, you haven't missed anything!