Something he would notice as he moves through is that the darkness seems to be thinner when looking towards the center of the 3rd floor, though it'd be unable to be seen what's behind it until moving through, which would still take a bit of time. In the meantime, his movements would feel somewhat weighted, as if wading through ankle-deep water.

Though I would also like to give people who haven't posted the opportunity to make a reply first! You'd be good to post again after [@Psyker Landshark] and [@Sifr] can get a reply out, though.