Cards in an opponent's graveyard are considered legal targets for spell/abilities with "target card in your graveyard" if they were put there because you cast them
Steps to reproduce:
Start a new game.
Using the dev menu, put Lightning Bolt in your opponent's graveyard, and cast Chancellor of the Spires. Use its triggered ability to cast Lightning Bolt.
Cast Call to Mind.
Call to Mind will target Lightning Bolt, even though it isn't in your graveyard. When Call to Mind resolves, it will put Lightning Bolt in your opponent's hand.
This behavior is not limited to Call to Mind. It also works with Eternal Witness, Archaeomancer, and most likely any other similar spell or ability.
It is also not limited to Chancellor of the Spires. It also works with Sen Triplets, Jace's Mindseeker, and most likely any other spell or ability that casts an opponent's card without exiling it afterwards.
I first observed this behavior during an ordinary game, so this behavior is not exclusive to game states that were constructed using Dev Mode.