Well ...

As others have pointed out in the context of gate screenings ... if the piece of luggage has gotten onto the concourse, it's already been screened --- as has (almost) everyone else who's gotten onto the concourse. If the danger posed by unattended luggage is that some Evil Item might be introduced into it, and the screening process has ensured that no Evil Items are on the concourse, isn't an unattended piece of luggage on the concourse safe?