We live in a 3 story house and, despite asking them not too, have caught previous au-pairs leaving ds downstairs while they do whatever in their room - on the third floor.
DS is old enough not to put his fingers in plug sockets or anything like that, but is quite capable of opening front door and AP on 3rd floor would have no idea a stranger was in the house, or that ds had gone for a wander on his own!
Leaving children unattended is quite a common one, APs just don't see the problem - afterall what do you expect with teenage/young women with little or no childcare experience and even less interest.
In all honesty, if these girls/boys could come to the UK and find work not involving children that would pay for really nice accomodation (computers, TVs, all mod cons), enough salary to cover heating, hot water, telephone bills, food, travel costs and so on and still have enough left over for clothes/social life - how many would choose to be au-pairs? About 5%, I recon, plus the lazy ones and those too incompetent to do a job with regular scrutiny.