Multiple detections, mostly black line

For the past 2 weeks, I get many random motion notifications. Some during the day and some at night. They will come in bunches of 5 or 6 and be every 2 or 3 or 5 minutes or something like that. No pattern. When I look at them, I usually get a gray screen with a black bar across the middle. But some simply say "no video" and some are video but nothing there at all. None of this has ever happened. Any ideas?