i had a chat with the owners yesterday, he said there are even pressure mats at the bottom of each window! apparently the system was installed in 1990, the company that originally fitted it is now long gone.
it hasnt been used for about 10 years, they stopped using it because it always falsed etc.
all the doors in the house have recessed contacts, quite a lot of the doors have been swapped etc. and contacts havnt been put back again.
quite a few wires stick out in various places so occasionally it goes off when hoovering etc.
it hasn't had any maintenance since the day it