Ran into the same thing with old plaster, the trick for me was ditching the alligator clips and using a good set of test leads with a ground reference on the panel can. Also try turning the receiver gain way up and sweeping slow, like an inch a second, because those buried wires in plaster go dead in spots. Cheap kits just don't push enough signal for the lath and horsehair walls, so you're not crazy, the tool is.

Sort of related but the Scouters in Dragon Ball Z being numbers instead of like a mechanical dial was a Toriyama thing where he just got tired of drawing the glass. The classic example everyone brings up is the Xenomorph having human skulls in its dome because they were cheap and the sculptor had them lying around. Also the little dots on Mickey Mouse's shorts in early cartoons were just ink splatters they stopped cleaning up. That Luffy scar thing though is wild because Oda actually drew the scar in the manga first, so the anime team would've had to match it anyway. Unless the accident happened before that chapter dropped and Oda saw the cel at a studio visit or something.