Alucard disappeared because he LET himself disappear. The sigil on the ground is a link between him (wherever he is) and that spot. I'll wager Alucard will reappear somehow before the series is over, may it be in a combat role or just some afterthought.
Alucard's whole deal throughout his whole existence in this series has been to die at worthy/human hands. Given that he's not actually dead (and wasn't 'defeated' by a human,) this means he'll have at least one reason to show himself one last time.