The guy is a assassin who sends people to a different realm between the living and dead. It would be kinda strange if he did not sense her. The skull in his forehead also makes me think he can maybe talk to spirits or something.
Maybe he can cross the realm itself too and move faster to different locations because of it.
I hope Natsu collapses and that Lucy has a chance to finally show how strong she has become now.
Acnologia has no interest in the current war, he is only after the Dragonslayers. Then he might fight anybody else.