It doesn’t need to be a phone company employee. Various apps report back your location and that gets monetized by the app company. That’s how “free” apps make money.
Do you have any doubt that Facebook would not hesitate to sell the identities of people attending a Trump rally to Antifa?
If someone planned to do something very nefarious, even more nefarious and evil than what the FBI does to turn public opinion against gun ownership, it would be a hoot to find a stray dog and duct tape the phone to it and turn it loose .Or better yet trap a coyote and turn that loose in DC with the phone on it