I think we use “someone” more often when we have a group of persons in consideration, but we don’t know for sure who the one is. While we use “somebody” when its not that important who the person is.
someone has left the door open
We don’t know exactly who left the door open.
somebody has left the door open
We don’t care who left the door open rather than the fact that the door is open.
How much sure are you about this rule on scale from one to ten?
Could you please refer to some official source?
If I answered your question with this post, use the motivating power of kudos!