In this case, you probably have the AccuTouch resistive technology and the front bezel of the monitor is in contact with the active area of the touchscreen. This acts as a constant touch.

The touchscreen itself may have a damaged spot. Examine the cover sheet for picks, dents or other damage.

If an external serial controller is being used, disconnect the touchscreen cable from the controller (the input to the controller, not the output to the COM port) while comdump is scrolling the data. If data stops scrolling, the screen is definitely damaged or the bezel is causing a constant touch.