Explanation is more simpler than you think. SEN is more like affinity towards Outer One's vision/perception. They normally see things that humans don't, like Star Vampires and changed human corpses that Grimm doesn't see with 0 SEN (regarding that Bandersnatch carried a corpse to her lair that is invisible with 0 SEN). SEN also makes people more vulnerable to mind/perception warping (regarding visions in the Hotel Poseidon, Mental Ward and warped vision of Red Hood, when we encounter her at the first time).
It is a fact, but it doesn't end there. Gods have ability to warp anyone's perception to their likings (check
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for example). And...triggers. For Grimm
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. Everything that is pleasant = Alice