The bangs

The eyes.

The glowing around the destruction caused by Shar worshippers.

The bangs or the eyes sure may just be coincidence, all three though? You'd have to have flunked selune cleric school to not realize something is up with Shadow

I'm still personally holding out for Shadowheart being a Dark Moon heretic and not a true Sharran, as that might explain why she remains so incredibly secretive even after you "discover" her allegiances.

They mind wiped her. Shes secretive because she literally knows nothing

I wonder if Selunite should not recognize that flickering light on her hand ...
After all, everyone "knows" what it is anyway.