People don't risk openly supporting Trump unless they have the kind of fuck-you money/assets that allows them to do so. Truth is that the tech industry is probably leaning toward the Democratic party a lot less than it actually seems simply because of the politically motivated discrimination you would face should you endorse the wrong candidate (or embrace the "wrong" opinions in public).
I have seen what happened to a couple folks who told "unappropriate" jokes in a private discussion to their friends while at a public event. They lost their job.
I have seen how tons of progressives (not to say most) are so entrenched in their own bias that they fail to realize that dissenting opinions to their beliefs are not morally reprehensible. And that their cultish attitude with respect to diversity is as stupid as the white nationalistic obsession of homogeneity.
Finally, I have seen how the democrats/progressives systematically demolish the lives of the people who oppose them. That's why I will remain silent. Post anonymously and take action with my vote. And that's why everyone at my workplace believes I am a staunch HRC supporter.
And before you accuse me of paranoia or to have a persecution complex I recommend you try to first figure out why so many people are behaving the exact same way I am: telling one thing in public (or to pollsters) and do another when they are protected by the opacity of a voting cabin.
edit: I posted this comment 4 seconds ago and it is already at -1. You read very fast!
However, telephone polling generally matches actual election results closely enough to assume that people feel a similar level of anonymity in the voting booth as they do in a telephone poll. The dismal polling results for Trump in the Bay Area are not because of secret Trump voters, it's because even conservatives don't want to vote for a candidate whose capability, sanity, behavior, and temperament are so clearly unequal to the job at hand.
This is the result of a direct and honest strategy of courting socially-conservative religious people who believe the country is being dragged "in the wrong direction" by people with "a radical social agenda".
While there are people who disagree with the general mainstream of socially-liberal belief in this country who aren't motivated primarily by religious beliefs, they are rare enough compared to the religious majority to not be worth mentioning most of the time.
Trump's results in the general election were bang-on with what the polls predicted.
The general could be different. But there's no reason to believe that it will be.
That said, I don't disagree that "polite" coastal society doesn't take kindly to alternative views.