Final answer:
John likely learned about Dog Beach during a long walk with Marcus and four dogs along the shore near the Golden Gate, as they were exploring the area and discussing the dogs' needs and places to take them.
Step-by-step explanation:
The passage suggests that John finds out about Dog Beach while on an outing with his dogs and a friend named Marcus. John appears to be accompanying Marcus on a walk to the Cliff House with four dogs that were let out from the little dog hospital where Marcus works. The Cliff House is near the Golden Gate, along the shore, indicating proximity to beaches, which could include the Dog Beach. While the exact moment of discovery is not detailed in the passages provided, the context suggests that the beaching area for dogs would have been encountered during their long walk along the shore, or perhaps discussed during their time together that afternoon.
John's attention to the dogs and their wellbeing, as described in Marcus's dialogue and enthusiasm about the dogs, reflects an interest in suitable locations for dog outings like Dog Beach. Therefore, it is likely John's knowledge of Dog Beach comes from his experiences and conversations around these outings with Marcus and the dogs from the dog hospital.