I'm going to throw this idea out here too:
Brian likely escaped by boat. (This theory requires 1 person unknown to us helping Brian) At Fort Desoto, during the night, he climbed on a tender that was prearranged (or hell, even an inflatable paddle board he brought with him and blew up at the campsite) and is now hiding comfortably on someone's boat. Someone the family knows. It's hard to hide on a boat by yourself. Dock neighbors can tell if someone is living on a boat and watch their movements, notice things ("Hey, their air conditioner is on but 'Bob' hasn't been around").
Even modest boats have AC and a toilet that flushes. You wouldn't even need to show your face outside. All a fugitive would need is someone (the boat owner) to pretend to go about their business, bring food and supplies back to the boat like usual, and none would be the wiser.
I thought the first thing the FBI would do is to check the fuel docks in the area to see if anybody made a large purchase in preparation for an outing. Or dock cameras. Or slip reservations? Or marina store cameras?
MOO
edited for spelling and MBIT (My Brian is Tired too!)