The defendant's disappearance from the neighborhood was discovered that same day.
You probably realize that the surface structure of your sentence (its present syntax) cannot be turned into the Passive Voice because the verb "to disappear" is intransitive. You may remember that such verbs do not take either indirect or direct objects, which is why they cannot be used in the Passive Voice.
Then, you need to rephrase the original sentence and include a transitive verb that will allow you to rewrite it in the Passive Voice. The previous answer changed your original verb into a noun and included the transitive verb "to discover" as a bridge toward the Passive.
If you wish to keep the original verbal form -"disappeared"- you may go to the impersonal passive, but you still need to add a transitive verb. The deep structure of the new sentence (its meaning) will remain the same.
The sentence I propose is just a variation of the one you were offered before, yet I like it better because it is quite straightforward.
- It was discovered that the defendant had disappeared from the neighborhood that same day.