To maintain security you really need a backup phone on you at all times that you can use to log out your other devices that isn't part of this chain. You additionally need someone else that you trust who has permission to do this, but that's just yet another attack vector.
Truth is, if someone wants to get at you they can and you can't stop them without being incredibly paranoid. So Bluetooth-based chain of trust, here we go.
Sild Road admin Ross Ulbricht aka "Dread Pirate Roberts" arrested with laptop left running, unlocked.
https://news.ycombinator.com/item?id=9545085
What unfolded next was a piece of improvisational theater. At 3:14 pm, DPR was typing away, writing to Cirrus. Just then, a middle-aged woman and man came toward Ross, ambling along in the kind of semihomeless shuffle you might often see in a San Francisco library. “Fuck you!” the woman yelled when they were directly behind Ross’ chair. As if they were a deranged couple about to fight, the man grabbed the woman by the collar and raised his fist.
Ross turned around for just a second, during which a hand reached across the table and grasped Ross’ Samsung. The petite, unassuming young Asian woman sitting across from Ross this whole time was, to everyone’s surprise, also an FBI agent. Ross lunged for his machine, a hair too late, as she turned like a quarterback for a quick handoff to Kiernan, who appeared out of nowhere—as instructed—to get the laptop. It took less than 10 seconds. From afar, Tarbell was astonished by the elegant choreography of the whole thing. It looked like the police procedural version of a tight jazz quartet.