The way this HA zone is constructed, it consists of two floors: the one used for the first steps where you locate the drop to close the portal and then the zone with the portal. The other levels are blocked off, basically don't exist. When collectibles spawn they can appear on either of the two levels you're working in. They can also spawn on top of pillars, broken walls and, for lack of a better word, stalagmites. I haven't found any that drop from mobs. On my last run I found three on the upper level and only one on the bottom level, and that's good because the see-thru-all-invis mobs were only on the lower level. Definitely worth having a rogue along for quicker searching, if possible!
Edited, Nov 5th 2015 9:05am by Sippin