Well, perhaps he made a mistake? Vic made a similar mistake and he is also far from stupid. Claims about the alarm have come from both sides on the forum Bridget but let's not forget that it was Jeremy who said it was installed 'after'. You were prepared to believe in it's existence only a few days ago, even though there was no evidence to suggest it was there post murders. You told me that just Ann's say so was evidence enough. Now you're using the opposite argument to accuse either Mike or Jeremy of being a liar - you can't have it both ways! 
GDS was quite specific, he didn't just claim that there was an alarm, he also claimed that it had been triggered at 5am on the morning if the murders. That would be quite a mistake.
There was evidence that it was there pre murders, but the evidence turned out to be a mistake on Vic's part (if we ignore the imaginative GDS).