Off market sales are not unheard of in the USA -- my neighbor's condo was sold off-market. They were talking to a realtor to prepare to sell, and the realtor put them into touch with a buyer that paid cash for higher than they planned to list it at. So it's completely possible for a home to sell without neighbors being aware of it.
Also, in my experience, many homes are listed on the realtors MLS systems before they hit public sites like Zillow.
I bought my house right after it was listed on my realtors MLS system -- I had already made an offer before it ever hit Zillow or before there was a For Sale sign in the yard.
