Moving the mouse pointer to middle of bottom edge of a screen will permanently move the Dock there. As far as I know, there is no way to disable this behavior. It's terrible.
For simultaneous multi-monitor, you're probably forced into using a full Dock replacement. There are a few options out there, but none have ever stuck with me personally.
"Motion is detected based on the amount of signal disruption taking place between the Xfinity Gateway and your selected WiFi-connected devices, so motion from small pets (around 40 pounds or less) can be filtered out while keeping you notified of large movements more likely to be caused by humans."