I think Margoo's comments about luck are definately relevant.

I'd imagine that separating that luck into 'crime scene luck' and 'post-discovery luck' might help.

Obviously the incredible bungling and the foolish obsession of the BPD could not have been predicted, but should we credit him with having been 'lucky' at the crime scene or should we think it more 'skill'?

If he left a sack and some un-used rope as well as perhaps leaving a flashlight at the scene one wonders if he is lacking in task attention abilities or if he is somehow being really clever.

Forensic awareness? Or just luck. Maybe he did leave footprints of some sort or debris from his shoes and that is why the carpet was cut from each side of the bed by the forensic types.

And if the ransom note's monetary amount was based on his favorite numbers, having that amount turn out to be the annual bonus was an incredible piece of luck rather than any sort of inside knowledge.