Originally Posted by
MSPeconomist
At some airports, from the gate area you see a herd of passengers walking behind a wall of glass. They are an international arrival and the glassed in corridor will lead to immigration and customs.
For the purpose of OP's question, this is the answer. Where international flights arrive, there are two layers of glass between the boarding-seating area and the outside. Between the two layers of glass is the corridor leading to immigration. Passengers boarding from those gates will go through two sets of doors and pass through this corridor for a couple of seconds. Of course airline/airport agents have to lock, release, or staff the doors correctly.