LED emission isn't purely narrowband.  It's likely that there is some near
IR emission.  I don't think you'd notice it under all the visible, with a
test card or IR sensitive camera.   The fact that the receiver picks it up
is indication that it is emitting something that the receiver is sensitive
to.

The receiver's filter is also not perfect, and there may even be enough
electrical noise to make it through at short distances.
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