First. This is NOT on this site. But you guys are good about finding lost stories.
Dead tree, from long ago. Possibly before any of us were born. Mystery author from the UK. I remember 3 distinct stories that I BELIEVE were the same author.
1. Locked room mystery set in a keep in Scotland. The detective is asked to prove that the old man did NOT commit suicide by throwing himself out an upper story
window.
2. A man is killed with a blunt tipped quarrel fired by a crossbow from inside a carriage. This causes a fatal heart attack. No one sees this happen in a public square in London.
3. A person overhears others talking in a restaurant or cafe, and realizes that it is about a murder that is being planned. I THOUGHT that the title of this was "X, the Unknown" but that is an old SF movie.
I originally thought the author was John Dickson Carr, but none of his stories seem to fit the bill.
Any ideas?