I've just entered my first mysterious portal on floor 4. Once in i've started to venture and after several linked rooms (about 5-6) it ended in a sack end. I've looked in all the rooms but I don't see any exit/portal to get out of this level. I wnt into each rooms several times trying to figure any sort of mecanism for an exit like hidden lever or any sort of passage but I can't find anything except respawning ennemies. Is there any way to get out if this secret floor?
I was going to report the same thing the other day, but turned out that the exit portal was on a tile next to the bottom wall of a room, and the wall blocked its vision. Try walking next to all the bottom walls to see if it is the same problem.
I don't speak from personnel experience, but you might need to knock some blocks around. Sometimes "Dis-placement Glyphs" spawn under other objects (Like mini fridges and such). Might be the same case with a portal.