Kai Team: It is private property (owned by a development company) but widely used for hiking and, as you point out, geocaching. Someone who lives on Route 173 appears to have taken it upon themselves to prevent anyone from parking along the State highway or walking on the property. I seriously doubt they are a friend of the owner (which is a development company, not a person) - just a busybody.
The property to the east of the bridge/stream on 173 is posted. The property to the west of the bridge/stream is not, which is why the cache description says to enter on the west side of the stream. That said, the busybody has put logs in the parking pull off (which is on public property - i.e. the highway right of way) to prevent people from parking there, and if they are now calling the police, we don't want anyone to get into a confrontation with the busybody or hassle with law enforcement. Although this cache also can be reached from a trail that starts on Glencliffe Road (which has a large parking area), it seems like it is best to archive it. Too bad, since it is one of the oldest caches in the county.