well the takeover part usually suggests that the sensor was already present before the install, or at least is not a sensor that comcast specifically supplies (thus comcast had to "takeover" that signal the device outputs and incorporates that with their security system)
however since you're mentioning about being confused on why the battery is already dead/dying, it sounds like that sensor was one that comcast supplied. and dead batteries on arrival seem to be at least somewhat common. although it could just be that all you need to do is take the batteries and put them back in. here's a link on how to change the batteries http://customer.xfinity.com/help-and-support/home-security/xh-device-battery-replacement