I've recently dropped 6 points in reputation but it does not show in the Reputation log. Is there a way for me to find out where or why that happened?
We had a similar case a few days ago on the Mathematics site. You might have received reputation from getting suggested edits approved, and the affected posts have been deleted meanwhile. That will reverse the reputation gained.
User @MartinSleziak wrote a Stack Exchange Data Explorer query which you can use to check; I see a couple of posts have been deleted (those have just a number in the first column, not a link) – those may have been deleted recently (I can't check because I don't have the reputation).