"Your wand," murmured Bellatrix, "I hid it in the graveyard, my lord, before I left... under the tombstone to the right of your father's grave...

He could have given it to her before he left (which would be evidence that he planned his 'absence') or she could have, say, taken a detour on the way to the Longbottoms' place to steal it back from the forensic Aurors and hide it.

Also, if the wand is still there I will be very disappointed in Dumbledore.