Yo no creo en las brujas, pero que las hay, las hay.
It wasn't the spider and it wasn't in the dining room (it's starting to sound like Cluedo!). The alarm detects movement in the kitchen (deduction after much empirical testing in the middle of the night). No signs of rodents or other fauna (arghhhhhh!!!!). The curious bit is that it only goes off during the night and not once during the day, when we set the alarm whenever we all leave the house. We'll have to call the alarm company.