Servant's on hunt still targetable by spells (Crimson Aegis, Power Surge)
I was running through the halls of my castle spamming Power Surge for mobility when I cast it on a "ghost". The silhouette appears as a Scholar woman. I have a servant that is a Scholar woman who is out on a hunt at the time this occurred.
My theory is this was the last location my servant was before I sent her on a hunt. The servant is hidden from the player, but remains targetable.