Unfortunately, unlike every other aid / safety feature on the Kona, the sensors aren’t on by default and are only activated automatically when selecting reverse. I, too, found this out the hard way. Having said that, my Kona makes so many damn beeps, bloops, bongs and ding dongs (enough to make me want to switch every aid off and drive it “dumb”) that maybe Hyundai thought a parking sensor beeping at you too would just overwhelm you.
There is a button near your right knee (where the lighting panel is) that activates the sensors manually.
Additionally, there is a setting buried deep in the cars control panel (annoyingly, as I recall, you have to scroll on that page to see it, so it’s not obvious) to make the sensors active all the time.
The Kona (not motability) forums are full of similarly infuriated drivers with the same issue, many (including me, I’m embarrassed to say) have shouted about this defect. It’s spectacularly annoying, but once turned on they stay on.
So many Kona owners have kicked up so much fuss over this that I don’t know why Hyundai haven’t changed the default setting in newer cars, it would cost them virtually nothing.