Grimrock

After you kill all the green creatures at the end of the level, there is a switch on one of the corridors that you now have access to. I flipped the switch and I could not find something on the level that is triggered by it, have I missed something?

If you are referring to the "end" of the level, after you get the scrolls, did you try looking at the gate that was blocking your way? If not, have to be a little more clearer on what you are asking...sorry

he's talking about a switch inside one the U-shaped corridor where the green creatures come out from. I also can't find what this switch toggles,

The switch just put the teleporters back (I think).

I thought that there could be something behind the teleporters, so turning them off would reveal some secret, but apparently there is none.
Or maybe you have to leave some items there? There is at least one teleporter that has different target when teleporting items and party.