Do you have a pool or something in your backyard? Maybe they're trying to get pics of your teenage daughter and friends around the pool. (That actually happened in Kentucky, only it was a drone. Homeowner shot it out of the sky with a shotgun.) My guess is, given your previous run in with them, that they still think you're making speed, or doing something illegal, and are trying to gather proof of it.
Try this. Go to Lowes or Home Depot, or get on Amazon, buy a really powerful spotlight, aim light right at camera. Now all they'll get is pics of a really bright light.