I didn't think of the Powerhorn Sirens.
They are triggers by the flashing of the security lights X10 address. So your neighbor using the same address would trigger any Powehorn Sirens you have.
The MS18 Motion Sensor checks in about every hour with the console and if the console does not receive any check in messages in four hours. It should trigger a Problem Message for the Zone it is registered to. If you are not getting any problem zone messages then it is within range of the console and distance is probably not the issue.
Is the MS18 set to mode 1 {instant} or mode 2 {delayed} triggering?
In mode 2 delayed it takes two motion detection's before it will trigger a violation of the zone.
The MS18 users manual {I don't think it is in the main manual} has a test procedure in it you may want to try it and see what it may show.
http://www.authinx.com/manuals/X10/MS18A.pdfThe X10 Wiki has a whole section on Security Consoles and accessories.
You may find some information there also.
http://kbase.x10.com/wiki/Security