In one chapter of the Discworld novel Night Watch, a man tries to kill Vimes from a nearby roof using a crossbow while the latter is enjoying a cocoa outside the Watch House (it makes sense in context).
Vimes is saved by a promising young Assassin, but the man's crossbow is not an assassin's weapon and Madam later remarks, "That doesn't sound like Swing".
Lord Winter has not heard of Vimes at this stage of the story and Carcer is not the type to let the dirty work to others: Do we know who sent this killer?