Projections require manually triggered refresh?

Projections do not require manual refresh following DML operations. A projection refresh is only necessary after creating a new one.

See:
https://www.vertica.com/docs/9.2.x/HTML/Content/Authoring/AdministratorsGuide/Projections/UpdatingProjectionsUsingRefresh.htm

Thanks. So, does this mean there's no need to check for stale projections?

No. It's very important to make sure all projections are "up to date".

This query should always return 0:

dbadmin=> SELECT COUNT(*) FROM projections WHERE NOT is_up_to_date;
 COUNT
-------
     0
(1 row)

Otherwise your AHM will fall behind and not advance.

See:
https://www.vertica.com/blog/ancient-history-mark-not-advancing/

what causes projections to become stale? sorry for asking this, but I am trying to find information from the documentation, but I couldn't find any.