Stolen bikes

Well, his face comes out quite clearly in that video, so let's hope it can count as evidence.

He seems to behave suspiciously from the start and is clearly there to steal a bike, and not just an opportunist, although getting an unlocked bike is of course an opportunist act. Perhaps he had tools in his backpack, too.

he seems like he went there with a mission, he eyed up the bike as soon as the guy got off it. He went inside for about 5 seconds, and no further than the doorway. I admit I'm guilty of leaving my bike like that, no more though.
He came on a Boris Bike and left it just out of camera resting against the gate, as everyone on twitter has suggested, he will be traceable through that. Assuming he didn't steal it or use a stolen card to get it.