“Mmh…” he didn’t respond with more than a grunt, only rolling his tongue in his cheek and squinting as he thought about what to do..

“Why can’t I sense his mana…” he thought to himself with a furrowed brow, “Usually there is some residual mana after a person makes it their own… but with him, there is none at all…”

Usually when someone activates a spell, there would be some ambient mana mixed in too.