How plausible is it that they were following people outside their group for union reasons, e.g. to call them for a union meeting, rather than anything more nefarious?

That is ok usage, but in the discussed case we already know that people followed were later doxed not called to discuss unions. Big difference

Also, we know that the people being tracked complained that they felt unsafe.

While they were being tracked or only once the case was being built and the tracking was revealed?

Who are these people? Managers? Would the union organisers have been tracking calendars to find times the managers are not walking the aisles so the unionists could talk to the staff about unionising?

In such a circumstance I imagine I as a hypothetical manager being coerced into a union-busting exercise could arrange to feel threatened by this calendar-monitoring behaviour.

As for the copies outside the office: we want to bring the union lawyers and membership folks around to talk about joining the union, here’s the manager’s schedule for the next week.

Believe nothing of what you read and only half of what you see.

It is every aggrieved party’s interest to claim they thought the world was going to end, because the only negotiating direction is away from that extreme.

I agree that in this case, the person subscribed to calendars for an arguably shady purpose, or at least a not work-related purpose. But I find it rich that Google created the feature, and then tried to describe it in a creepy way, especially with how Google approaches privacy for their users (roughly, data the user hands to Google is fair game).

The issue here is not about the feature or existing possibility here. The issue is about people being followed and then they were doxed. This is creating a hostile workplace. Google makes investigation, gives out warnings, bad behavior continues, result is inevitable

Unionizing and protest organizing is just a background noise at this point