Why did Robert Scott leave his tent in the antarctic?

Reasons by Scott left his tent are unremarkable: it was necessary to move in and out of shelter.

You may be thinking of Captain Lawrence Edward Grace "Titus" Oates, who suffered from extreme frostbite on his foot during the return trek from the South Pole -- with Scott, whom he disliked.

Oates was apparently concerned that his presence in the team would slow them down and make it harder for his mates to return to safety. One night, then, he left the tent with the comment "I . . . may be some time." He never returned, nor was his body ever found.