Thank you, David.
I've compared my "My fry first" with your reply, I think what you say is more concise.
in (1), the first impression "something" gives me is the police are entrapping "the man", while in (2) is, "somebody" is more open to discussion, but it needs some unusual context to bolster the whole sentence.
That said, suppose the right context is given or is established, what is the usual way to say what is suggested by the "something" or "somebody" in each case?
1. The police haven't confirmed if the man has _______ to do with the murder.
A: The police haven't confirmed if the man has a particular thing to do with the murder. ---- ?
2. She lost herself in doing homework so that she didn't hear ____ knocking at the door at all.
A: She lost herself in doing homework so that she didn't hear a particular person knocking at the door at all. ---- ?
This is what the two mean to me at first sight.
So we need some particular context respectively.