Update: thank you everyone! user @Today has provided a great link of a discussion that suggests the correct answer is where being an abbreviation of, whereas as a replacement of since, hypothesized in these comments.
As I love archaic definitions, I’m more convinced to now that this is the answer!
Especially since the question originates from one weirdo using “where” instead of since.
https://english.stackexchange.com/questions/338694/is-it-ever-appropriate-to-use-where-instead-of-because-or-since
Like “Where we knew he was heading to Chicago tomorrow, we got on the first plane heading east to intercept.”
“Where we knew where the safe was, we began to cut through the wall in the corner behind her desk.”
Thanks
I’ve never heard this in California… but “since” can imply any kind of causal or logical relationship, while both your examples seem specifically related to physical proximity. So is it possible the usage you’re noticing is constrained to that kind of context?