Motion Sensor Alarm?

The motion sensors are not immune to problems or sensitivity.  They are heat-sensing, so even if your dog got close to the shelf/sensor, or jumped onto the couch, it might have picked up.  Other things that can set them off:  heating/ac vents, ceiling fans, sunlight moving across windows, drapes moving, even a bug on the lens.  If your dog typically gets on the couch to look out the window or something when someone leaves, could be she is setting off the sensor if too close to it- but also rule out interior temperature changes.   You can also set the sensitivity setting to lower (a little switch inside the sensor, take off bracket, it's visible).

@pitzerboy, one other suggestion.  I have 10 motions and, now, no issues.  Did in the past and found drawing a floor layout with the motion sensors, air ducts, fans and windows shown on it did a great job of showing potential issues, short and long term. This methid even let SS support and I, after several attempts, solve an issue with one of them we both didn't know about.  (Long story short, a kitchen motion was picking us up entering through a door and sometimes set off the alarm. Moving the sensitivity as coltmaster suggested look care of that issue.

I did have to reduce the sensitivity on three of them and move two. Now they work great.