Wearing the knife under his jacket? Hardly convenient, IMO. It's not a schoolboy error. Almost no one who is going to use that knife as an offensive weapon is going to wear it on a conventional belt, IMO. People who carry them for self-defense sometimes use the belt loop, but typically have the knife at the small of their back, because it's long enough and hard enough to impede regular movements of running, twisting, leaping, etc.
Everyone I know who carries a knife for self defense wears it in their jacket. It's a custom where I live. And yes, there are quite a few knifings among the homicides of my region. There are even jackets that allow someone to carry a few knives in a series, and jackets designed for both right-handed and left-handed people (BK is right handed, so easy to get one from Amazon). Carhardt is a known brand, there are many others, including ones that have pockets for other useful items that one wants easy access to.
Wearing it on his belt would have been uncomfortable and I believe he wore a longish outer dark jacket, essential to his kit because he needed to have something that would obscure and perhaps block blood spatter from his other clothes, underneath.
If he failed to take off the jacket or to put protective coverings on his driver side front seat, then he's in a real pickle, forensically.