Hmm, that doesn't seem likely then - although I'm at a loss on what else it would be. Are you sure about the relationship between turning off the outside camera and the motion alerts? Is it possible that was just a coincidence? I'd be inclined to turn the camera back on, and see if they return consistently over multiple days. If they are IR motion sensors, I guess it is possible that there is some IR leaking through the window. That would likely require open curtains, or otherwise a fair bit of light leakage. IR is just light that you can't see, so if you can see visible light through the window, IR is coming through as well. If there are motion sensors close by that window, it's possible it could set them off.
In terms of the camera position, I was thinking of first just trying to turn the camera a little (but keep in view what you want to see) to see if the IR leakage through the window reduces and maybe avoids the issue.
