I'm going to test this, but if you have set to fire once, it should have a specific date and time for that once to run. I have set it for 21:00 on 17/03/2012 in just over 20 minutes time. We will have to see if it generates any ghost scans at 21:00 after that.
EDIT: I have also set it to wake up the system to start the scan just in case I'm away.