The Blue Rock Springs Park and Lake Berryessa crimes have one distinctive and advantageous feature, in that we can use the phone calls to generate his possible movements, using the two points of contact and search into reasons for the 'lost time' after his departure from each crime scene. Obviously his attacks were fraught with danger, but he would further gamble with his freedom, by ultimately making further contact at each payphone, conversely this risk would be greatly diminished had he disposed of the 'smoking gun' first, in the case of Blue Rock Springs, the 9 mm Luger, with a change of attire a possible consideration. Had the Zodiac Killer lived in Vallejo, as the majority of people contend, a return journey to parts of this region, including American Canyon, with a ten minute turnaround, would take the killer back to the payphone at the Springs Road and Tuolumne intersection at about the correct time of 12.40 am, when the call was logged. His journey back from Lake Berryessa, in a southbound direction through downtown Napa, fortunately for us, sets him on a collision course with American Canyon and the heart of Vallejo once again.
The difference between the second and third crime is that he used a knife, in the close contact attack on Bryan Hartnell and Cecilia Shepard and undoubtedly would have been covered in blood. His 'smoking gun' was greater in this case, that being the costume, gloves, knife, blood transfer, along with the unused handgun, and traveling from the crime scene, directly to the payphone was not plausible, if the timeline was to be maintained. He could have simply placed these items in the trunk of his car, but like the Blue Rock Springs Park attack, the unaccounted time suggests he made a deviation. Concealing the items in his car or taking these items home, still leaves us with 20 minutes spare time, that needs to be explained. If, like his previous crime, his first intention was to remove any incriminating evidence before placing the phone call, we have to ask, where did he go that fits our timeline and was there anything we knew about the Zodiac Killer that may give us a clue.
It has been considered that the Zodiac Killer may have been a hunter, hiker or fisherman, possibly all three, on account of the specific key words he used in his correspondence with police, the locations of his crimes and the weaponry he appeared to have access to. But can we use this line of thought to fill in our missing 20 minutes.
Lake Berryessa is 40 miles from Vallejo taking the Interstate Route 29 through Napa and a marked change in distance from Lake Herman Road and Blue Rock Springs, with many locations en route, suggesting that he carefully selected this as his destination and more than likely somewhere he was familiar with, or at the very least had visited previously.