Looking at your map- the gates to Organ Pipes park close by nightfall ... I think the terrain there would be difficult. But plenty of open space around it. Similarly, no access to Brimbank after dark and I'd assume similar issues to being seen near where they live. That said I have been told bodies have been found in Woodlands Historic Park though, and I assume the same issue with the closed gates, so obviously this isn't an insurmountable problem. Arundale and Annandale, I would consider them reasonably well trafficked. Open land, but plenty of through traffic, not isolated. That said so is Oaklands Junction ... I am being too picky.
I'm thinking Lerderderg could be a contender though I'll be frank, I have lived here all my life and no one has ever mentioned going there, without looking at the map I would draw a blank about the place personally ... maybe because people drive up to Macedon instead?