Exactly, what is an FP for the sandbox, since you are talking about the auto-sandbox, it isn't making a determination that what it is flagging is infected.
The file system shield (FSS) is the first avast shield to come across the executable file and depending on what is known about that file, is it digitally signed or in the avast persistent cache, what location is it in, also probably using the Emulation function in the FSS would pass that off to the auto-sandbox for action/response.
That may be to run it sandboxed or to allow it, of course you can change the Auto-Sandbox mode in the settings to Ask rather than Auto. That way anything passed to the sandbox lets you know the recommended action, which you can change and you can allow it and 'Remember my answer for this program' if you are confident that there is nothing wrong with it.
