I don't think you are being paranoid. Leaving windows open in a bungalow overnight or even daytime in rooms that you are not using, sounds risky.
It is often said that would-be burglars look specifically for open windows during heatwaves.
Plus you could be on very sticky ground insurance- wise if anything did happen and you were deemed not to have taken reasonable operations.
That said, aren't open windows during the day counter productive in this type of weather? I know it can vary from one property to another, but in most cases I would think you are far better keeping the heat out rather then letting it in.