Well well well. Are you new in that neighborhood? Was your property perhaps vacant for some time that tossing plants over the fence could be done casually?

Hmmm. I think I'd discuss hosta first chance with the neighbor when you are both out working in the garden. To keep it "neighborly" perhaps mention that you are a hosta fan and note they also grow hosta. Any divisions would be appreciated if they ever have any available. If you feel inclined to do so, give them your phone number so you can come get them. I would not mention the ones you "found."
Perhaps they are new to the area also, or a yardman/child/teenager threw them over the fence without the homeowner's knowledge. As long as they are healthy, consider it manna from heaven. :)

Moccasinlanding I posted this post to amuse you all. :) My neighbor is my daughter. She bought her house as an estate sale as did I.

Last week my daughter was having a cook out. DH went into her house- took a cold beer and walked back over to our house. One of her guests was livid! He wanted to confront that "neighbor" and my daughter told him "that's my dad". Her guest kept saying "no- it's your neighbor- I saw him." lol

But back to the Hostas- I am really surprised they lived through such a cold winter with little protection. I just had to save them. :)

undulata albomarginata .. albo=white.. and margin=margin ... the white edged one ....

and undulata univiitta .. aka the white centered one ...

both.. pure driveway hosta... without soil.. all summer long ... cant kill them if you wanted to ...unless someone sold you one for $100... then it would die on the way home ... lol

neighbors are idiots... lol .. except yours of course... being spawn and all ... lol ...

ken