I'm watching the 2021 Australian Open men's singles final between Novak Djokovic and Daniil Medvedev.
In the first set, with the score 3-3 and Djokovic serving, Djokovic won one of the points when Medvedev hit a ball just long. I'm pretty sure Medvedev did not challenge the call.
Then while Djokovic was preparing for his next serve, I saw the screen behind him display "CLOSE CALL" and show a Hawk Eye replay that declared the ball went "OUT". I didn't know that they use Hawk Eye even if a player didn't call for a challenge. (I have seen Hawk Eye replays on TV before without a challenge, but I thought those were only shown to the TV audience, not also shown in the arena.)
So what would happen if the Hawk Eye replay in the arena showed that a call was wrong, but no player challenged the call? Would the original call stand or be reversed?