The following observation may be reading a little too much into the Zodiac's wording, but the Vallejo Times-Herald communication is the only one of the three letters he used the past tense regarding his mailing of the letters. He used the phrase "Here is a cyipher or that is part of one. the other 2 parts have been mailed to the S.F. Examiner + the S.F. Chronicle," whereas the San Francisco communications used the present tense: "The other 2 parts are being mailed to the Vallejo Times + S.F. Chronicle" and "Here is part of a cipher the other 2 parts of this cipher are being mailed to the editors of the Vallejo Times and SF Examiner." If he was a resident of Vallejo, and was going to post all three communications somewhere in or near the Richmond District of San Francisco, which he did, then subconsciously on his future return to his residence in Vallejo, he would have posted the other two communications (to the Examiner and Chronicle). That is because he is associating the Vallejo Times-Herald with his home location. The letter he is sending to the Vallejo Times-Herald is close to his residence, so he labels the two San Francisco communications containing the word Vallejo, as are being mailed, because he is projecting further ahead. This subconscious use of words may very early on have told us exactly where the Zodiac Killer lived- somewhere he knew that couples were readily available over the weekend, before he returned to work in San Francisco on Monday morning.
If the killer lived in Vallejo and traveled to San Francisco weekly for his occupation, where did he work?. There are plenty of reasons to believe the Zodiac Killer knew the Presidio Heights area fairly well. If you are going to murder a taxicab driver in an affluent, built-up area, in the heart of San Francisco, it would have been advisable to know everything about the area and your likely escape route in advance.
There are two ways to enter San Francisco by vehicle from Vallejo- either by the Golden Gate Bridge or Oakland Bay Bridge. Either journey from Vallejo to the Richmond District is approximately the same distance. But if you entered via the Golden Gate Bridge every day, you would certainly have every opportunity to become accustomed to the roadways in and around the Presidio Park area. This is the area the Zodiac Killer may have left his vehicle- somewhere on the fringes of the park, by West Pacific Avenue or Lyon Street- towards the east.
A man fitting the description of Zodiac was seen running into the Julius Khan playground minutes after the murder of Paul Stine- a description that virtually matched that of the three teenagers and Officer Donald Fouke. If we contend this wasn't the killer, then we have to conveniently dismiss this independent eyewitness testimony as erroneous. Even Dave Toschi believed this was a viable sighting, as search dogs and police vehicles were stationed alongside the Julius Khan playground.
A passage in Robert Graysmith's book Zodiac appears to corroborate the premise that eyewitnesses at, or near Spruce Street, observed a man running into Julius Khan playground, and tallies with the October 12th 1969 San Francisco Chronicle article. Here is the excerpt: "The detectives heard from neighbors that a stocky figure was seen dashing across Julius Khan playground and into the dense undergrowth of the Presidio. The dog patrol units, seven of the best search dogs in the country, gathered at the front entrance of the Presidio and were deployed one at a time in various directions. Armstrong and Toschi considered the possibilities. Had the killer gone quickly through the dark woods and emerged from the Presidio at Richardson Avenue, and taken Highway 101 past Fort Point onto the Golden Gate Bridge and vanished into Marin County." Richardson Avenue leads onto Highway 101 and the Golden Gate Bridge- a route the killer would be completely familiar with, had he traveled this way to and from work each day. Even if he didn't travel this exact route, if he worked in the Richmond District area, he would surely be familiar with the surrounding area and his best escape route from the intersection of Washington and Cherry.
This area and vicinity is an extremely significant area when it comes to the exploits of the Zodiac Killer in San Francisco. This can be seen in the Google map diagram below.
On April 20th 1970 the '13 Symbol' cipher arrived at the San Francisco Chronicle stating "I am mildly cerous as to how much money you have on my head now. I hope you do not think that I was the one who wiped out that blue meannie with a bomb at the cop station. Even though I talked about killing school children with one. It just wouldn't doo to move in on someone else's teritory. But there is more glory in killing a cop than a cid because a cop can shoot back. I have killed ten people to date. It would have been a lot more except that my bus bomb was a dud. I was swamped out by the rain we had a while back." In the letter the Zodiac Killer was referring to the murder of San Francisco police officer Brian McDonnell on February 16th 1970, two months earlier, when a bomb packed with fence staples exploded on an outside ledge of the 1899 Waller Street, Park Police Station in the Upper Haight neighborhood. This location, bordering Golden Gate Park and the Richmond District areas, is only 1.45 miles south of the Paul Stine crime scene.
In the following correspondence, the 'Button' letter, on June 26th 1970, the Zodiac Killer suggested he may have been responsible for the murder of Sgt Richard Radetich a week earlier. Officer Radetich was gunned down by three shots from a .38 caliber revolver at point blank range through the driver side window of his vehicle, while in the process of serving a parking ticket. He was sat in his police car near 643 Waller Street, San Francisco, in the Lower Haight District, where he sustained life threatening injuries and subsequently died 15 hours later.
643 Waller Street is only just over a mile from the Golden Gate Park and the Richmond District of San Francisco. One can see the clustering of these Zodiac 'events' within the northwest corner of San Francisco, and around the Richmond District area. Even the 'Dripping Pen' card mailed on November 8th 1969 was supposedly mailed just south of this area.
Nobody can definitively pin down the likely profession of the Zodiac Killer, but did it reside somewhere in this region- somewhere he could mail his communications to and from work. The question is always asked: Why would the Zodiac Killer mail a noticeable amount of his correspondence from the vicinity of where he worked?- that would be stupid. The answer probably lies within the geographical profiling term 'the least effort principle.' That, when a human can exert the least effort in accomplishing a given task (with little risk), they probably will. Obviously, he realized that the ability to trace a letter or card back to a particular mailing point is practically impossible. Only to the area the letter was franked.
The final observation on the map above, is the locality and proximity of the three golf courses to the Richmond District- bearing in mind the golf course adjacent to the Blue Rock Springs parking lot. Could the Zodiac Killer have worked at one of these locations, or did he play golf on a regular basis, taking his scorecard from the clubhouse into the very communications he mailed.