You say you can't get near them but if you have not been in the garden, how do you know? It might be that the owner can handle them. I hope they have somewhere safe at night but have seen all sorts.
chap I work with has had 85+ guinea pigs, he kept all the girls in a shed overnight and every morning he let them out to roam the back garden it was like a herd of mini buffalo's :lol:
I'd be tempted to leave be, if they've been roaming for 2 years chances are they have a safe haven or two
They must have a safe place/bolthole. The fox would have picked them off by now.
When I lived in London, they wouldn't have been safe in a hutch, let alone running around.
I now live in a more country-fied area, less foxes, but did see matey running down the road at about 20 to eight the other morning.
I wouldn't risk it, sorry. Seems odd they are in everyone's garden though, I love Gunieas, but I'd be mad if they eat my plants.