How much power does the average computer use on stand by or hibernate? Will a motion detection program be able to detect anything if the system is on stand-by?

I have been wondering about the possibilities of catching a thief in the act. What would be involved in starting the computer when the door is opened (or voltage through something)? Could this be accomplished by an alarm system that warns the thief? Instead of (or in addition to) warning or inducing a siren, which is normally ignored, the computer would be powered up by a trigger and start recording video or take pictures.

Just a few things to ponder...

thanks
Scott