I believe there was a security system panel on the wall in the mudroom, next to the door into the garage. The natural place you would put the control panel. Perhaps she only used it at night, but not during the day as she was coming and going. Or perhaps the kids knew the code and FD found out about the code from them.
I keep thinking about that door from the back yard into the mudroom. That would have been right next to where the Wed BBQ was. That would be another place he could have entered the house undetected and waited for her in the garage.
Here is what I think is her security system panel in the mudroom, on the same wall as the door into the garage:
https://photos.zillowstatic.com/p_f/ISmy8wsbu2mqqj1000000000.jpg
This is the door from the mudroom into the back yard. FD could have found a way to get into the mudroom from this door and wait in the garage for her to return.
https://photos.zillowstatic.com/p_f/ISu8aeqegmg1jj1000000000.jpg