You put a camera somewhere, set the frequency on the camera and a computer connected to a monitor, and when the program is running the monitors will show the view from the camera.
NOTE: The camera can be changed to any imaging device.
a camera is the only thing I know. />
This sounds super difficult to implement...or maybe more like impossible. But that's if you want an actual camera. If, on the other hand, you just want to program a computer to make use of the various sensor devices already in the game to build a visual display of mob activity in a defined area, that's already possible.
I think that an IR ranged motion detector isn't a bad idea...there probably already is one in the peripherals section. But I'm not exactly hungry for it.