After I killed the Gargantua with the air strikes, I wanted to explore the tunnel I had to run like hell through (cool sequence btw, I felt like he was on my ass the whole way, with a car being pushed out the tunnel along side me). The tunnel doors appear to be open. They are bent with a large gap between them, but an invisible barrier prevents reentry.
Tunnel should either be blocked fully (lame… I notice a TON of that in BMS. Backtracking is prohibited almost everywhere) or the invisible barrier removed.