Maybe this is kind of an anti trap? It does not do anything (or deal some damage) if you fall for it. It does nothing if you disable or dispell it.
But Harley activated the real function by trying to bypass it. And this real function is to open a portal.
This would explain a lot.
- nobody (TE) notice any kind of spell effect, since usually, there is no effect (or some kind of effect you would expect from a trap) No plane shift or other kind of travel.
- the color of the floor changed after 'bypassing' the trap. This seems to be more likely if the dungeon leads to another pane/dimension/location NOW.
- even if TE could notice the runes (I'm not sure they can) they might just burn a dispell on it in order to render it inactive. This is especially likely if the trap has some kind of visible effect (like damage) when activated normally.