If you think of how a hog would be with a wild nest – i.e. wouldn’t use it again, probably to avoid parasites, such as fleas and ticks – it isn’t surprising that they don’t immediately use the boxes again. Despite our efforts of cleaning them out.
However, if I were you I would leave them where they are for a while. Some hogs wait until they become ‘part of the furniture’ before they use them, and you never know, a hog might be planning on moving in any time. I think you were lucky last time round that someone moved in so quickly.