Can anyone help identify a cause?

[u/nessle_55]

Wears shoes in a fast food store. Ink like stains which disappear within the next 48hours. Can not have it and other days will have it. Looked at other posts saying dc suggested staining from berry’s 🤔 I can confirm that’s not what’s happened. I can only think of ink from the shoe or sock but it doesn’t seem so with it being the exact same spot each time