I've noticed before that my character has gotten stuck in "trap" rooms where a teleporter goes in but there's no way to teleport back out, but I confess I haven't seen one where there's a teleporter out but no way in. Keep in mind that teleporters needn't necessarily be those brick wall textures; they could be random objects in the dungeon that you have to click on. Without a rigorous search I couldn't tell you if the scenario you described is even possible. It's possible that it's possible (if that makes sense) because a lot of the dungeons are randomly generated, but I wouldn't think that would normally happen in practice.

You might try the map trick, which is where you look at your map and use it to check for hidden doorways; you'll be able to tell where they are because you can see the doorway behind the door even while it's closed. Other than that I can't help you.

I haven't been able to find a pattern, honestly. Floating skulls in the middle of corridors are popular. They could also be 2D sprites sitting on a table, an altar, I've had one where you had to sit on a throne to get it to teleport, it could be a [&@%!] box sitting on the floor. Just follow your adventure gamer instincts and click on everything (making sure that you save frequently, of course)