When the hedgehog travels furtively over the lawn (10)
Here's another unexpected, unusual nature image. The "hedgehog" is moving "furtively," or cautiously, across the grass. It's nighttime – why is it being so cautious? It could connect back to the "dewfall-hawk" of the previous stanza.
Watching the full-starred heavens that winter sees (14)
Wait a minute – "winter" can't "see" anything – it's a season, not a person. It's being personified (go to the "Symbols, Imagery, Wordplay" section for more on that). The neighbors who are "watching" the stars are being subtly, or implicitly, compared to "winter," which also "sees" the stars. So maybe people and nature aren't as separate as they might seem?
"He was one who had an eye for such mysteries (16)
The speaker of the poem is also connected to the neighbors and to the "winter" through the same idea of watching the stars. He's a person who "had an eye" for "mysteries" like stargazing.