These free floating planets could come in from any angle — not a lot of chance that Saturn or Jupiter will be in just the right place to catch an earth bound one.
The reason they are not located near a star is simply to avoid problems associated with stars ~ to wit, unshielded radiation.
Some of these civilizations STOP star formation in their galactic clusters ~ elimination of new stars avoids serious waste of resources as well as major accidents such as rogue black holes.
Our problem is this ~ let's say somebody living in one of these things decides he/she/it/they/what wants to visit Earth ~ so zippo bingo they arrive next door ~ and visit.
How do they do that? Well, we don't know, but any civilization sufficiently advanced to build a Jupiter sized Dyson Sphere probably has all the knowledge they need to relocate on whim.
We should imagine that about the time we can "notice them" they may be interested in "noticing us".
So, maybe this weekend?