Enclosed cats may not be safe

Cat owners, if you have a six foot tall privacy fence it is still able to be scaled by a cat. I usually let my one cat lay on the chair on the porch. They could also sneak to the bottom of the steps for some grass. One morning recently I opened the door to let the dog out and found a stray cat on the porch. As soon as the door opened he flew out into the yard and in one swift jump he was over the six foot fence. The dog didn't even see him, but of course was following the scent with confusion.
I wasn't giving my cats the feline Leukemia shot thinking they were safe inside the fence. Now they are making me question whether they may have been exposed to it.
Don't know why the cat came into the yard. It may have been hunting early birds.I don't feed my cats outdoors or the dogs either.
I really don't think my cats can get out of the fence since I know when they sneak out and watch them. The cat that sleeps on the porch is 15 and not very active that he would jump that high.Still I am not going to allow any more trips outdoors in the yard for a while.