The safest way for them to be in your yard, fenced or not, is with your eagle eye on them at ALL times!!!!
I do have a fenced yard but even so, I always keep an eye on them when they are out there anyway. They usually don't stay out too long anyway unless we are out there with them. Point in watching them every minute being that I let them out first thing this morning and out of the blue, Toby found a space in between the gate and the fence that he could fit through....and off he trotted. Shocked
the heck out of me and out through the sliding patio doors I went from where I was watching them(just a few feet away). I thought for sure I'd be chasing him down the driveway or across the neighbors yard in my nightgown! But luckily, I just opened the gate and he trotted right back in..........this time. For the time being we put a yard statue in front of that space, but will be blocking it as soon as we can. And of course.....they will still NEVER be left outside unattended or unsupervised.