I was recently asked whether companies in Silicon Valley ask a job candidate's prior employers for a feedback about the candidate. Although the overly simplified (to the point of not being meaningful) answer is "no," it's actually complicated and nuanced enough to merit a longer discussion.
At the Silicon Valley companies I have worked at, we always check references. And we often check informal (backdoor) references. References matter. Will they make up for an otherwise lackluster resume or interview? No. But will they tip the balance of two otherwise similar candidates? Yes, they can.
A backdoor reference is someone in our common network of colleagues, friends or acquaintances that knows you (the applicant) in a professional setting.
What we won't do, unless you explicitly agree to, or suggest, it is check with your current (soon to be previous, if all goes well) employer about, you for two main reasons:
Most large companies in the valley don't "give references". That is, all of them will confirm prior employment, and dates of employment, and sometimes some other objective facts like job title, etc. But none of them will give a qualitative reference about performance, etc. There's simply too much liability at stake.
Out of respect to candidates, we don't want to alert a current employer that one of their employees is looking for a new job.
A little more about #1: If you permit us, we will talk to someone at or from your current company. But since most companies prohibit giving a reference on behalf of the company, it's really a "personal reference" who happens to have knowledge or impressions of professional you. And we trust that if you tell us whom to talk to, you trust them to be discreet about your job search.
A little more about #2: Since the most we will do without you explicitly saying we can contact someone at your current company, whom you recommend, is use the third party service to verify employment, you company doesn't know you are out there looking -- since we could be a bank calling to verify your employment information, related to getting a loan, or something similar, completely unrelated to a job search.
But now about those backdoor references: Obviously sometimes our networks don't overlap, but for anyone who has been in the Valley a while, there is almost bound to be some connection. In these cases, one must tread lightly and use discretion -- and that is what we do.
We don't randomly find some connection via LinkedIn and cold email a connection of a connection asking about you. It would be unlikely to generate useful responses and would often enter the territory of tipping off you current employer about your plans.
But if there is someone we know, whom we trust, who can provide or discreetly gather some data points, that's very useful -- and often even more powerful than the names you provided us as professional references since we can calibrate the answers of people we already know and we don't get the biased selection of relying on names you provided (whom we assume you have selected as the most likely to provide the most positive reference).