I think I’ve shared this before but when I was on my journalism course a retired detective came in for the day to talk to us about working with the police, how investigations work etc. We asked him if he thought it was possible to come up with the perfect crime, and he said the two key things that made it incredibly hard to catch a killer were
a) If the victim / killer had no prior relationship
b) If the murder was committed outdoors
So feel free to factor that in.