I certainly wouldn't argue with anyone that the X10 MS10A is "frustrating" - Here's a few things I've learned with my installation - maybe they can be of help.
Pay attention to the drawings of the "field-of-view" of the MS10A.
Remember that the sensor detects heat moving within its "Field-of-view". And contrast is important - the farther apart in temperature the moving object is than the ambient air, the easier the sensor will pick it up. If it's 50° in the room and you walk by naked, the sensor will trip sooner than if the temperature in the room is 80°.
No motion sensor is pet-immune, unless your pet is cold-blooded or dead.
To block off the "floor" from being "seen", try ½" of masking tape on the bottom of the lens. If that doesn't work, add another ½", etc.
To make your sensor even less sensitive, try a layer of waxed-paper over the entire lens.
Positioning is everything - mine have been tripped by hot air.
1. Heat rising from components of my home theater - especially when something gets warm and its fan turns on.
2. Heat from my HVAC system - STAY AWAY FROM VENTS or RADIATORS! Read the part about contrast above - If you setback your heat at night, say to 60°, then at 5:00 a.m. have it come up to 75° - the contrast between the ambient air (60°) and the heated air coming out the vent (120°) is greater than if the ambient air is 70° - therefore the sensor will "see" better and farther. Remember also that most electric heating systems are 2-stage, where an additional set of heating elements come on only when the "contrast" is greater than a set level, OR when the system has been calling for heat (and not able to satisfy the thermostat) for a set period of time.
Pay attention to your activity monitor - save & study the files. Let your house get cold, then crank up the heat and watch the monitor - you'll see if there are sensor trips.
Getting the MS10A's positioned correctly is the hardest part of the setup. And these are different than the EagleEyes and HawkEyes - and for different purposes - the MS10A should generally be mounted lower than the Eye's.
And remember you can use tape or business cards to "narrow" or "focus" it's field of view. If you are looking for motion in a small area, focus the sensor to that area.