Im speaking from my own experience. 2 infinities and 2 Nissans
Brake pads might look ok but when u change them light will go off.
If he goes to a dialer they will say same thing.

Ok, given that something like 75% of braking is done by the front brakes, you're suggesting that, over the span of only 20,000 miles the rear brake pads may need replacement? Also, why would the light go away when he lifts off the gas if the pads are worn? Of course the dealer will suggest a service that makes them money. It's easy enough to check to rule it out, though.

If it's not a fluid issue, I'm guessing there's a sensor that's on its way out.

It Might be front pads. Need to see how much meat left on them.
To me it always happened on the rear. But i usually go thru a 2 brake sets on the front before changing rear.

Still, pretty unlikely at 20k miles, whether it's front or rear. He might as well check since it only takes a few minutes to pull a pad out and look.