There are several stages to post deletion, I think. At least in the case of deleting your own post, it will show as a deleted post for some configurable number of hours. Then, the system marks it for hard deletion, and it is not visible to the public. I’m not sure if the owner can still see it, but admins can. After another period of time, the system actually deletes it, and it will exist only in backups, or web archives, or personal captures.
Some forums will be run in a very transparent way, where the mods will engage in a conversation about a deletion, and perhaps even PM the poster a copy of their original post. Other forums will be run in such a way that the poster may not be aware that their posts have been subject to moderator action.
If a forum has a public policy of transparency, and you find that some action has happened without transparency, you might want to contact the admins in case they have a misbehaving moderator, or in case something else has gone wrong.