There are several posts on this topic already but I feel like this is a really elegant solution for detecting bodies in a room with usb webcam. Works by taking a snapshot of the background and comparing new frames of video against this snapshot. A more detailed examination and demo video can be seen on my blog here...

http://www.customarttools.com/blog/2015/12/14/a-simple-way-of-detecting-the-presence-of-a-person-in-a-room