Motion Sensors Unreliable

@dpickut2 Full disclosure, I am a former ADT customer of 8 years too and now a SS customer of almost 9 years and have recommended and placed over 30 systems.  And do call out SS when they fall short, here in the forums and directly when they deserve it.

As far as the motion sensors, I have one in my garage, six on the first floor and four on my 2nd floor...and they all work fine. Did they at first? No.  I had them on secret alert for a while (about 10 days) and placed them in my home and moved a few so that did not cause issues. All are on either low or medium sensitivity, away from heat/cooling ducts, ceiling fans and direct sunlight. No false alarms on SS3 for over three years.

I would suggest you call support and work through your issues with the motion sensors.  One last thing: When I got my first system (SS2) I got a floor plan of my house from the sales brochure, marked air ducts, and where I was going to put all of my sensors. Scanned and emailed to support as I was having issues with  my alarm going off before I could get into the kitchen and disarm the system. After 2 calls, the agent found the issue: motion sensor at other end of kitchen (20-24 feet away) would ocasscaionaly pick my wife or I up entering from the garage.  Moved the sensitivity switch to medium approx 8+ years and no issues since.

If you get a chance post your outcome here, and, good luck.

Oh, the motion sensors trigger for a reason, although that reason isn't always obvious. Captain mentions some of the "gotchas".

I switched recently from SimpliSafe to Safely that was recently launched in UK (safelyteam(dot)com). My biggest reason was that SimpliSafe currently offers minimum home automation capabilities and does not seem like an expandable system. Otherwise, Simplisafe works pretty alright. It seems like Safely system, although does not have many device integrations currently is poised to offer a lot more in the coming months. Plus I really like the app and its responsiveness etc. Not to forget it's a Zigbee based system that gives a lot more options in the future for me.
If your Simplisafe system is within the return time frame, you could seriously consider looking into Safely.

I interfaced my old ADT microwave-based motion sensors (actually made by Honeywell) to the system via entry sensors.  This setup has worked flawlessly over a year's time.  

I'm an electrical engineer with decades of experience with digital electronic equipment.  The SimpliSafe motion sensors are a very poor design.  There was nothing unusual about my setup -- actually had them in the same locations the ADT sensors had been for more than 10 years.  

Thanks for the responses, however.  I'm going to look at the Safely system, as I've experience some reliability issues with SimpliSafe.