The notice type that is a field of a question appears to be optional, yet it is not marked as "may be absent"?

For example, when I request questions with a filter that has the notice field checked, not one seems to actually return the field: https://api.stackexchange.com/docs/questions#order=desc&sort=activity&filter=!Pw)uU(3C7Kb3SII2)E)3YNmxmpOu5r&site=stackoverflow&run=true

Is this a mistake in the documentation or am I misinterpreting it?