I forgot to return to Insight Park - I'm a horrible detective.
Maybe we have another hint in that prophecy that the White Dragonborn, the maybe not so dead Julikar Caeziryn, is using a disguise spell to impersonate other people (which wouldn`t be difficult for a sorcerer):
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Turning the bark over, he reveals to you a scene rendered in bloody sap. It shows you running down the streets of Baldur's Gates, chased by Flaming Fist mercenaries who hold a wanted poster featuring a crude sketch of your face.
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When you inspect the bark more closely you note that your shadow takes on the appearance of a dragonborn.
I'm not sure how the physics of a Disguise Self spell work, but
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If you use this spell to appear thinner than you are, the hand of someone who reaches out to touch you would bump into you while it was seemingly still in midair.
Maybe it works the same way with light, and the shadow does not match the illusion?
(He doesn't use Alter Self because it's not in the game )