We know for a fact that the Hogwarts wards do not raise an alarm when they should, because they did not detect Draco being under a Blood-Cooling Charm. And we also know that Voldie had a better idea regarding those wards (whether he actually had said wards in place around Draco is debatable, but still, he had the idea).

So I think it's extremely probable that the wards he has to detect his own death are more efficient than the Hogwarts wards, and he's currently riding Bella's body and kicking himself for once more not having just used Avada Kedavra.