fox attacks

Now, perhaps I am being just a bit obtuse here, but the fox attacks that take part inside people's houses....Why does no-one say how the foxes have gained access?  Is it warm weather and the foxes are wandering in through open doors and just exploring?   If not, just how do the foxes get into the houses?  It's not like they have opposable thumbs and a set of skeleton keys.
These foxes, I suspect are getting in through windows left open, or maybe cat flaps.  The window thing should not be a problem as in this day and age, when most people have UPVC windows...it is possible to lock your window in a vent position, which is not enough to let intruders of any kind in.  In fact UPVC windows can be locked in the vent position.
As for cat flaps...surely if you know there are urban foxes around, it would make sense to put in a cat flap with a collar sensor, so only the household cat could get in?
Just things that went through my mind when I read the article about the fox attacks.