I would recommend getting your cats vaccinated if you are going to let them
outside.  While no other cats may be around while you are out, they may
visit when you aren't, and your cats may pick something up.  Rabies vacc (if
you are in U.S.) is mandatory, FVRCP for feline distemper is recommended and
I would suggest FeLeuk for feline luekemia (your vet may want to test to
make sure they aren't positive now.

JoJo

If they have lived for at least tree weeks in the new place they will find
the way home, don't worry. You can't really stop a cat from roaming. But you
could give him a real fright when he goes past the border: Hide and throw a
water balloon or similar at him. A light hollow chain is a good thing too,
bangers near him, whatever. There is no limit for creativity...
(When I do this with the martens I have to use heavier arms though. My
friend laughs at me and says it's like a comic to watch :-). I have got rid
of them for the time being using an ultra sound device which humans cannot
hear but it disturbs the martens and keeps them from destroying my roof and
s****ing everywhere, but that's all besides the original question.)

Carola