What is the word for "currently" unworn?

The problem with your edit is that the context you've provided implies that the shoes have been worn before (you've seen them before on his feet) but that they are not currently being worn (they're obviously empty given that they're lying on the floor). All you have to say is this:

On the floor lay a pair of shoes. I had seen them before, on his feet, from under the bed where I hid, as he paced and looked for me."

No need for any adjective modifying shoes.

If we consider dress in the context you've provided, you could say any number of things, e.g.:

"On the bed lay a dress. I had seen it before, on her body, from the closet where I hid, as she paced and looked for me."

Again, given the context you've provided, no need for any adjective modifying dress.

Addendum: You edited your context to read:

"The ___ pair of shoes kinda makes me lonely."

For this I think empty works. That the emptiness of the pair of shoes "kinda makes me lonely" implies that at some point the shoes were not empty, that someone -- for example, someone who mattered to you -- had once worn them.