It's not that uncommon. You have people making crazy threats (which they probably don't intend to carry out, but it only takes one..) all the time, and the police won't do anything but take a report because frankly, that's all they can do. So you end up at a certain scale having to investigate some of these things yourself.. by hiring probably somewhat sketchy ex-military types.
Yeah, having a threat intelligence and investigations team is very common for big companies.
If nothing else, it's probably safe to assume that someone like Mark Zuckerberg or Larry Page regularly get emails or other messages saying things like "im gunna go to ur house nd kill ur dog and ur hot wife"
Law enforcement doesn't have the resources to investigate every one of these, absent a credible threat of it actually happening, so corporate security / executive protection exists to, well, investigate.
I"ve seen Mark's security when he's out and about on campus. They're reasonably discrete about their jobs, given that on any given (pre-Covid) day there were probably hundreds of business and personal visitors on campus.
Personal security for CEOs (never mind billionaire CEOs) is hardly anything new. If I were Mark, received the amount of hate/threat messages he does, and had a young daughter, I'd do everything short of hiring the 82nd Airborne to follow me around.
Taylor Swift once said "I don't have security to look cool. I have security because people want to take me home and chain me to a pipe in their basement."
She wouldn't last five minutes chained in my basement because she'd start singing and I'd throw her out! Next if be looking for the person who put her there because that is not an acceptable practical joke! I'm glad she has security.
Retired police officers & feds of various flavors aren't uncommon in California. Their retired LE status lets them carry concealed without issue, even in downtown San Francisco.
For some positions, the ability to carry concealed was mandatory (e.g. the CEO's driver/bodyguard). Related aside: the CEO I'm thinking of wasn't allowed to drive himself in order to avoid any stock hits if they whapped into a kid running into the street after a ball (or what have you).
I've had some experience at a corporate security team at a fairly large (non-tech) firm, including internal and external investigations, and I can confirm I never saw or heard of anything remotely like this. Any time we did any sort of research into a non-employee (which was very rare), it was because they were explicitly making threats, or because we received reports from employees that someone was directly harassing them.
And typically it was just to gather information to provide to law enforcement, when necessary, and to provide information to our physical security team when we thought there may be some increased risk of a belligerent person trying to come to our premises. Very definitely not to take matters into our own hands and act like a dystopian quasi-paramilitary cabal terrorizing random people with obsessive psychological warfare campaigns. It was the total opposite; we were constantly walking on eggshells to do as little as possible and to defer as much as possible to law enforcement, as we should've been.
Never once did I see the slightest inkling of a desire to go after critics of the company from anyone on our team or from management. We would get some visibility into that from dashboards showing mentions of the company on social media and stuff like that, but we'd just ignore or occasionally chuckle at (or agree with...) any criticism we saw.
I'd like to think this hyper-paranoid, authoritarian, power-tripping attitude (let alone the abject psychopathy and psychosis) isn't too common in corporate security teams, but I have no clue and only have my n = 1 sample to go off of. And maybe the distribution is different for tech companies dedicated to rapid growth/monopolization, compared to more traditional, "boring" companies.
