On Sun, 30 Mar 2014 17:07:34 +0100, Mike Barnes wrote:

My Brother-in-law has just bought a house where the previous owner had
died, so no-one to ask questions of. Inside there is a small (about 3x2
inches) yellow plastic box mounted on the wall, with a small black
plastic key (about 1.5 inches long) in it. To the left of the key hole,
still part of the box, is what looks like it might be an indicator of
some sort, but whatever position the key is turned to, the indicator
stays the same.

It is positioned by the front door, and there is another one in the
main bedroom.There are cables running to the boxes, but can't trace
them without a lot of removing of fixtures/boards etc.

It's probably a "panic alarm" box. There will be a large button to press
when you detect a burglar breaking in or already in. Pressing the button
will activate the alarm. The key is to reset it and would normally be
stored somewhere else.

If the previous occupant was elderly and frail, it may well be the "base
station" for a necklace-style fall alarm.