I'll close this question by reporting that I've found the gearbox grounding brush on the Kona/Niro(classic)Ioniq series. It's on the tail end of the input shaft and appears to ride on the side of the bearing inner race. There is a brush on the motor output shaft as well but it's the circular carbon filament type, not purported to be particularly effective at reducing voltage potential.
These simple splash-lubricated gearboxes, similar in design to the Leaf, have been plagued with noise problems since 2019 and my best determination after following this issue for 6 months is that it's due to poor retention of break-in wear particles leading to bearing damage. There is an internal non-serviceable magnet but evidence indicates that it's not effective, perhaps due to the type (ceramic) and/or location in a turbulent area. Circulating particles turn black after being continually crushed, losing whatever magnetic attraction they once had and taking their toll on rolling-element bearings. Owners find black oil after less than 5,000 km from new! And not just one or two, 100% who have have done an early oil change, so far over 20 owners.
Some of us Kona owners have added aftermarket neodymium magnetic plugs which seem to work much better and they are less subjected to the oil windage being flush inside the threaded boss. I've checked mine and they are doing a great job.
I'd post photos if they were hosted here.