Yeah. I don’t know anything about scannning patterns of detectors… Just throwing out an idea. If you cover the detector and the light’s still there, it’s not it. Could be hard to do if it’s a combined detector and camera.
Spiders on my IR cameras look quite like that. And they often travel directly up and down while starting a new web, and then around as they complete. The web itself isn’t always visible in my experience, unless it’s close to the camera and the IR emitters are really illuminating it. I have to go out occasionally and knock down webs if they are too close, as the IR they reflect completely blinds my cameras.
When you are looking out the window in that one video, try shining a flashlight out there, and see if there’s a spider hanging out.
Otherwise, it would be weird for somebody or something to be shining an IR beam on your house. I guess you could go outside when it’s happening, and fluff some baby powder in the air, and see if the camera shows an actual illuminated beam revealed by the cloud. That might show the direction of the source.
Get your smartphone, activate the camera, point it at the suspicious place, (you can see IR light on the image displayed from digital cameras) if the point is there, you can use a white paper to lay it under that dot, now move it in the direction of the unknown source (away from the door) untill you find that source.