According to invoices and other documents reviewed by CNBC, as some Tesla employees attempted to form a union in 2017 and 2018 within the Fremont, California factory, Elon Musk paid a consultancy firm - MWW PR - to observe employees on a Facebook group and their social media accounts.

The firm monitored discussions around unfair labor practices in Tesla, in addition to a lawsuit on sexual harassment, according to the newly-revealed documents. 

Records that confirm Tesla's surveillance of its employees have been years old, and the relevance of the revelation has increased now, especially after Musk's known intention to take over Twitter.

If the deal is completed, Musk will become interim CEO of the social media company on which he relied, throughout the years, to promote his companies, mock enemies, whistleblowers, journalists and the US Democratic party.

Read more: Musk's Starlink, Space X arms of US empire & war machine

According to the records, Tesla paid the firm to not only monitor the Facebook groups, but also to conduct research on the organizers to create labor communication plans, media lists and pitches based on their union. 

Musk does not like unions. In 2017, Tesla fired Richard Ortiz, a union activist, followed by a tweet by Musk which violates federal labor laws. The National Labor Relations Board demanded Tesla to repeal the decision, also demanding that Musk delete his tweet which they argued threatened workers' compensation. Tesla was not so cooperative.

Jennifer Grygiel, a Syracuse University associate professor who focuses her research on propaganda and social media, told CNBC that: “Any organization can engage in ‘social listening,’ using publicly available social media data to gain insights for product development, or to understand voters, public and employee sentiment and more.

“But there are laws in the US that protect the rights of people to organize. If you’re a PR firm, or a manager who has to infiltrate a semi-private group? That’s dishonest. And I doubt Tesla would send a PR firm to figure out how to support workers involved in organizing.”

Three ex-Tesla employees told CNBC that back then, they were warned by their colleagues not to accept their bosses' friend requests on their social media networks, nor to join Tesla employee groups on social media unless they knew everyone in the group personally. 

Musk threatens to fire Tesla execs if they don't return to office

Bloomberg reported on Wednesday that Elon Musk has allegedly threatened to fire the executive staff of Tesla Inc. if they do not go back to working 40 hours a week. 

An email sent by Musk to the company's executives had the subject line "Remote work is no longer acceptable," made it clear that he will fire those who want to continue their working-from-home routine, according to the report.

“Anyone who wishes to do remote work must be in the office for a minimum (and I mean *minimum*) of 40 hours per week or depart Tesla. This is less than we ask of factory workers,” Bloomberg cited Musk as saying in the email. 

Musk, in parallel, said that Tesla executives must return to the main office, and not a remote branch office, according to the report.