What made Mary notice the round doorknob with the lock?

Mary has already found a key. Now she needs to know which door it opens. She's certain that the key will allow her to gain access to the closed garden, but she still has to find out where the door is. The next time she ventures outside to play, Mary jokingly asks the little robin to help her locate the door to the closed garden. After all, it was the robin who showed Mary where the key was, so perhaps he might like to go one better and show her where it fits.
Much to Mary's astonishment, the robin happily obliges. He flies over to the top of the closed garden's ivy-covered wall and lets out a glorious burst of song. Suddenly, a gentle gust of wind rushes down the walk, brushing aside some loose ivy trails that had been covering the garden wall. And there, in all its glory, revealed by the magical breeze for Mary to see, is the knob of the door to the closed garden—the secret garden, no less.