get a RTC shield with a SD card slot
once you have your first event, data log the time and "HIGH" or "Detected" or some such
as for the 'not detected' condition, that will be a timed condition ?
if there has not been any movement is "X" seconds, then the condition becomes low.
then write to the SD card the time and "LOW"
there are lots of links that will walk you through this.
spend a few minutes reading about the RTC and SD card and data logging.