Why it's the location of Alviso Adobe Community Park. The Museum on Main's excellent website has an informative and educational article on the park, but it does not offer either a map, location, directions, or address. There is an offhand mention of Foothill Road, but it's a long road, so that wasn't very helpful. The website is Web Link
The City of Pleasanton website is no better. There is a link to "Parks and Trails", and when I clicked there, there was a further link: "To see a list of park locations and amenities, click here". When I clicked that link it downloaded a pdf document listing all the parks in Pleasanton as supplied by the Parks and Recreation Commission. Unfortunately, the Alviso Adobe Community Park IS NOT ON THAT LIST!
Well, now this is rather curious and mysterious. So I have to guess that either the park:
1. Doesn't really exist or
2. Its location is being kept a secret to prevent over-use.