Phantom avast Sandbox in my Flash Drive

So what was going on here?

Yesterday I went to download some Apps over at my sister’s house. I stuck my Flash Drive in the computer and for the first time ever, I noticed that in the root directory of my Flash Drive, there was an “avast Sandbox” folder. Normally all that is supposed to be there is a Documents folder and a PortableApps folder, an Autorun file and the PortableApps.com Start shortcut.

What the … ??? That computer doesn’t even have avast … never has. It has Norton in it. So, I thought, "Well, I guess that folder was put in there by avast over in MY computer for whatever reason.

However, once I got here to my computer and stuck my Flash Drive in it … there is no “avast Sandbox” folder there in my Flash Drive’s root directory any longer. What’s going on here? Any insight? It had never happened before.

I have avast 2014 9.0.2021 and Windows XP.
My sister’s computer where the Phantom avast Sandbox folder appeared has Windows 8.1.