A Shiny Morning Key

I set out across the wooden floorboards to see what sits near the front door. Resting right by a tall sneaker on the lower shelf was a shiny brass key attached to a small metal ring! I leaned in and gave the cool edge a gentle push with my gloved hand. It slid across the smooth shelf with a cheerful clink, leaving a tiny scratch-free trail in the morning light. What kind of secret do you think it unlocks?