Whether Zodiac viewed the search or simply was able to listen to it (the sound of the responding vehicles etc) isn't really important. What is important to establish, if possible, is if he was there in the area to be able to see or hear the search, or whether he got the information of the search from somewhere else. We know that Zodiac isn't slow in responding to Officials when they publically state that they don't believe him or demand more evidence. "In answer to your asking for more details about the good times I have had in Vallejo, I shall be very happy to supply even more material" he replied when hi involvement in LHR was questioned. And this time he'd claimed in his Oct 13 letter that he was in the park, which Chief Lee had refuted and called Zodiac A Liar. He responded to this with the same attitude of being happy to supply more material about the search.
One possible theory to his whereabouts which could explain him only hearing the search vehicles is that he was in the back garden/yard of 3712 Jackson. This could be argued for based on several known facts.
1. It is the last known place that Zodiac was seen, with Don Fouke stating that he last viewed the suspect turn onto the driveway of 3712 Jackson.
2. Zodiac would have needed to be close to West pacific Av. to hear the responding vehicles, the rear of 3712 backs right onto W. Pacific Av.
3. There is fence that would obscure Zodiac's view of W.P.A but would be the ideal location to enable him to hear it, without him being seen.
Below is a photo of the front view of 3712 Jackson, and a gate can clearly be seen allowing access to the rear.
front-view-of-3712-jackson-st.jpg