In the 1950s, the Northampton Museum
(home of the "World Famous Shoe Collection) began to receive reports of shoes that had been found hidden in buildings. The shoes, usually discovered by people doing renovations or repairs, were concealed under floors, inside walls, in chimneys, above ceilings, etc.
Eventually the Museum received enough of these reports that they realized the concealment of the shoes wasn't an accident, but rather that hiding shoes inside a building was an ancient, deliberate practice. Ever since then, the Museum has kept a record of all concealed shoe finds (the "Concealed Shoe Index"). As of 2012, the index had over 1900 reports of shoe concealment from all over the world (but mostly Europe and North America).
image source: wikipedia
The Museum curators aren't entirely sure why people hide shoes inside buildings, but the leading theory is that it's a form of protection superstition, done to ward off forms of evil such as witches, bad luck, or the plague.
Shoe historian June Swann, former keeper of the Northamton Shoe collection, notes in a 1996 article about concealed shoes
that there are all kinds of weird superstitions associated with shoes:
there is much recorded on other shoe superstitions, which are rife wherever shoes are traditionally worn. They are symbols of authority, as in the Old Testament. They are linked with fertility: we still tie them on the back of wedding cars. And they are generally associated with good luck (witness all the holiday souvenirs in the shape of shoes). But most of all they stand in for the person: it has been a common practice from at least the sixteenth century to at least 1966 to throw an old shoe after people ‘for luck’.
Why the shoe? It is the only garment we wear which retains the shape, the personality, the essence of the wearer.
Just recently, maintenance workers at Cambridge University found a 300-year-old shoe hidden inside a wall
And earlier this year, a Michigan family discovered 53 pairs of shoes behind a wall in their home
— concealed there since the 1970s. Though in that case, it was theorized that the hidden shoes weren't warding off bad luck, but instead were evidence that a previous owner of the home had a shoe fetish.