Motion Sensor Alarm?

Last December while away on vacation, we had three instances of a Motion Sensor going off.

Our dog sitter was using the system, and this is the first instance of anything like this happening.

We have the motion sensor on a book shelf, upside down, so that the dog typically does not activate the sensor. The only way we think she can activate it is to maybe get up on the couch?

I have the original basic system, and I can review the log and nothing looks out of the ordinary. Every time the alarm went off, it was about 10 minutes after it was set.

I do have a camera for that room, but I don't have a chip to capture events. I'm going to work on getting one in the future.

Anyway, I was just curious if anyone has had any problem like this before.

Thanks!