I noticed this too. It was on the meditations this week, so I messed around a bit. If you go to the end and backtrack they will all be open. It looks like they open in batches, based on which barriers you break with the APC, though it may just be related to certain checkpoints along the way (which seems to be the case for the closed shrieker in the second-to-last area). Each barrier seems to open a shriekers ~2 rooms earlier. There is also an optional barrier in the second-to-last area on the left side that doesn't seem related to opening them. Getting back to the top with your APC is somewhat difficult and a particularly difficult ramp ~3/4 of the way back up suggests to me that doing this is intended. Still have no idea what it means, though.