Yes, officially this is a documentation bug. From the Absent Fields doc:
Fields that are not always guaranteed to be set... are indicated in the per-type documentation. Any discrepency(sic) between observed behavior and the documentation should be reported on Stack Apps.
(Emphasis added and note that "discrepancy" is mispellerized. ;) )
notice obviously should have the may be absent flag.
I eyeballed all of the other properties of type question, as well as type answer and type user. I didn't spot any other obvious discrepancies with the may be absent flag.
Re:
The notice type, that is a field of a question, appears to be optional, yet it is not marked as "may be absent"?
Optional and may be absent are separate dimensions.
- "Optional" -- means you must craft/use a custom filter to get the property.
- "May be absent" -- means that even if called for by the filter, the property will still not be returned if the property's value is null for that particular question.