Firelocks are invisible occasionally.

[dalmationer]

Thin firelocks seen around z-level transitions/stairwells appear to occasionally be invisible. I have also observed normal firelocks being invisible, as they create a black block as if they were a wall, and you can't see the firelock itself.

Observed on Northstar, specifically both can be seen by setting the fire alarm off on west side of the station, on a z-level above departures.