Not sure if that fits what you want - or if you know it already. Spike leaves after the beating he took. S/other mentioned I forgot but possible B/other but it will be spuffy:
Haunted by kantayraSummary:
Seven years ago, Spike fled Sunnydale and his abusive relationship with Buffy. Now, he and still-best-friend Dawn are working together in their own demon hunting agency. However, when they're called out to investigate a chain of supernatural murders at Cascade Mountain Lodge, they discover that Buffy's on the case as well. Will they be able to solve the mystery before they become the next casualties? And what does the solution have to do with the events of seven years ago?I've read it as a WIP back then; good read.