Pairs

I am slightly alarmed that I am missing a shoe.

Not a pair of shoes, but a shoe.

I'm not a huge shoe person anymore (I used to love shoe shopping and I don't know what happened), so I don't have a lot of shoes. And this is my nice pair of party shoes. This is not good at all.

I know I had them this spring when we went on the cruise. And I'm relatively sure I have either seen or worn them since then. How does one shoe disappear?