I am thinking some sort of superpower/magic but I'm not picky what exactly. It comes out because he either has some power that comes in handy (healing obvs. or he can just teleport Claire in the flat) or because after learning about the heartbeat he goes 'so you knew all the time about me being X?' ('No Foggy. I didn't. But thanks for giving me a heart attack. That's exactly what I needed')

Yeah so how would that have been? Foggy would still be pissed about the whole 'what about Karen and me if you were found out?' But wouldn't be able to complain much about the 'you kept that from me'-front