Data you could have observed but didn't

You're running a study that involves keeping records about humans. You have a spreadsheet with rows for each person and columns for height, weight, and eye color. You get pretty far in your study and then realize you sure could have used hair color too, but shoot, you didn't think of that in advance, so you don't have that data. What kind of data is hair color in this example? It's not an observable because you didn't observe it. It's not a latent because you totally could have observed it, if you'd thought to do so, it's right there.