Hm, I had some problems with foxes. My hutches are outside, and after a fox broke into one hutch I put better wire on them and fenced them in. Since my confidence in strictly passive defense is limited I put a trap door into the regular fence door, combined with an acustic alarm.
I always have a wildlife camera somewhere outside so that I have an idea what is going on.
Last fox that showed interest was a healthy, young one. They are smart, and cautious. Dont know if that video still works, it doesn't on my computer:
http://www.rcmovie.de/video/28bd272f6b506529569b
You can hear when the rabbits noticed him. Anyway, he came back, got trapped- and didn't like it. I hope he permanently associats domestic rabbits with getting trapped, I never saw him again.
The first fox, the one that killed 5 of my rabbits, a severly mange ridden old one with barely any fur left, wasn't as easily deterred, when I met it inside the fence face to face I shot it.
Living in a quiet valley with the woods just across the street has some drawbacks, but a fence and really sturdy hutches help a lot. I'm not sure the advice local hunters gave me is sound, but what the heck, it definitly doesn't hurt to pee around the perimeter from time to time. But I need to keep my eyes open, and react if necessary.
Imho the rabbits need places to hide in the hutches, not being able to completly get out of sight of whatever is at the door has 2 problems - the rabbits can be scared into shock, and the predator will try everything to get them as long as it sees them.