I just think burglars and other con people are brazen and confident and rely on not being challenged. I once heard a story of someone being robbed in their flat when they were at home( not Forest Hill); the burglars broke in during the day, totally ignored the resident, and just took the TV and other stuff before closing the door behind them. Probably just as well for the resident but particularly unnerving. I also years ago lived in a student flat when two men walked in and unplugged the TV and took it away whilst my flatmates were sitting watching it, thinking that they were repair men of some kind. Another flatmate held the door open for them on their way out.
Unfortunately I also watched some burglars take stuff out of a house across the road some years ago during the day not realising I was watching a robbery. It just looked as if someone was moving out.

The best protection I think is to know and be known by your neighbours so that anything unusual registers more easily and that they feel ok about calling the police if something out of the ordinary happens. Good neighbourly relationships always go a long way and benefit all parties and interested interference is probably the best defence against random opportunists or planned burglary. I would also be careful about all of these mystery callers who claim to be selling something but instead may be casing your joint.