and if weekly scan fails it will try it again automaticly. you can close it in Policies > Antivirus and Antispyware Policies > Administrators Defined Scan > edit your weekly scan options > Shedule > Retry the scan within unCheck box.
for example: if your scan schuled time in wednesday 12:00 pm and you install new sep in friday. Sep said " oh i was have weekly scan in wednesday and i didn't do it. thats why i'll start it now" than start scan immediately.