In Vinewood Hills, there is graffiti on a brick wall that reads 1807, as well as an arrow pointing to a house.
This is a reference to the criminal Charles Manson, who lead a small cult known as the Manson Family and was responsible for orchestrating several murders. The 07 represents the number of killings Manson was convicted of.
The arrow points at a midcentury ranch house that bears a close resemblance to 10050 Cielo Dr, the house where Manson sent 3 people to kill everyone inside. The house is located in Banham Canyon, this could be a reference to the location of the real house in Benedict Canyon.
At night, a party sometimes occurs at the house. If the player walks into any of the partygoers, they will collapse and die instantly. This may be a reference to the murders committed in the real life house.
A few streets below there is a very similar house, this could be a reference to 'the twin house' which was a house located at 10048 Cielo Drive that was nearly identical to 10050.
Another idea is that 1807 could be a reference to 187, the California Penal Code for murder.
Other players theorize that the arrow may not be a reference to Manson. It could be pointing to another location, going southwest. If that is the case, the 1807 could indicate a time, which is 18:07 or 6:07 PM.