The autosandbox process is controlled in the first instance by the file system shield (FSS), the suspect.exe file is scanned before it is allowed to run. If it were infected, it could/should be detected by the FSS, so one reasonable thing in its favour is it hasn't had a definitive detection.
However, the FSS checks other things amongst those a) is the file digitally signed, b) its location and what it does (this is done in the emulation check). these can trigger a suspicion and it is this suspicion that results in the recommendation to use the autosandbox.
Now the user can accept this decision and run it in the autosandbox or have it run normally and to Remember the answer for this program. Provided of course you are familiar with the program and that it is clean and of course that you intentionally initiated the program.
