If you have good footage, you might be able to use a Difference Matte:
http://help.adobe.com/en_US/aftereffects/cs/using/WS3878526689cb91655866c11...
You can probably build a clean plate for this out of existing footage if you didn't shoot one.
Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA