[Spoiler Alert: If you have not read Voyager or would prefer to not be spoiled about season three, it is best to skip this post for now.]
Lucky fans and a great photographer made great finds in September 2016 at both at Dunure and at Midhope Castle (Lallybroch) and came upon filming at both locations. Chris Saint-Martin took close up photos of both Sam Heughan and Caitriona Balfe filming at Dunure. And producer/writer Anne Kenney was spotted in the wild at Dunure.
https://twitter.com/_tiger_lil_/status/825965015849119744
Source: Chris Saint-Martin, New Glasgow Girls, OutlanderCast Blog