I assure you S13, it is extremely loud and annoying. And no-one to my knowledge assumed that it's being nicked.
It probably needs the sensitivity adjusted, but I can't work out why it's only done it in the mornings when they are out?
I will politely inform them when they get home. If it happens again next week though, I will have no option but to raise it at the next neighbourhood watch meeting. Once those old buggers get their false teeth into it they will wish they'd had it stolen.