what does mislaid key refer to?

The "mislaid key" passage is from Chapter 7 of Animal Farm. In this passage, the narrator describes how the animals have been so accustomed to vilify Snowball that they will blame any disturbance on his interference. As described, they believed that a locked store-shed and a missing key was the result of Snowball's devilry, so convinced that they continued to believe it even when the "mislaid key" was found. The overall point is to illustrate how ideology corrupts and how the masses will ultimately follow whatever makes them feel safe, even as the cost of truth. It's a theme that resonates throughout the novel.