If we don't spot anything, there's really nowhere else for a fall to have taken place outside the elevator shaft - being pushed down the stairs doesn't make much sense. He's a sixteen year old boy, not a decrepit octogenarian. He'd have recovered his balance unless he were passed out.

[Which, she supposes, could still be an option as well.]

...You kept a close eye on your chloroform this week, I hope?

[Susan will follow Rapture up towards the entrance to the rooftop and poke about as well.]