I don't think it's likely that any member of the Community left the sled there. Unfortunately, there's no evidence within the film to suggest who might have left it or why.
For a clue as to what's happening, and why that scene seems to not fit, I'll have to share a semi-spoiler from the end of the book:
In the book, it's intentionally left unclear as to whether Jonas
actually found a sled, or if he imagined it in a delirium as he froze
While I personally believe that he died, the ending is described
vaguely enough that it's also possible that he just stumbled across a
village of free people, and found a sled that one of their children
had left unattended. In the book, he only slides down a hill, not a
mountain, and he winds up someplace populated: he sees lights from
windows, and hears singing and music.
The movie throws that ambiguity out the window by showing him passing through the barrier on the sled, then showing emotion returning to the people of the Community.
The book didn't have any scenes outside of Jonas' point of view; so, when the filmmakers tried to mimic the end of the book, they unfortunately rendered it incomprehensible by showing us what was happening in the community at the time.