Mike,,it sounds very much as though Jeremy wasn't aware it was there,,and may have been fitted latterly,after threats were made to the family.
That being the case,,had Jeremy known,,he wouldn't have taken any chances in entering the property,,nor would he have stayed behind,hiding anywhere,,instead of leaving at 10pm when he said.
The whole point in having a panic button installed,,is not to tell anyone,,so that if there was an appropriate time to use it,,then it's depressed.
Sheila may not even have been aware of it either,as word about its existance would soon spread,and it then defeats the object of its use.
I can fully understand Neville not having had a burglar alarm,,as they tend to advertise the fact that there are valuable items within.