This question was motivated by this 2018 article by the Electronic Frontier Foundation that discusses the findings from the Mark Rettenmaier case.
For a bit of background on that case, this article at ocweekly.com explains
Rettenmaier is a prominent Orange County physician and surgeon who had no idea that a Nov. 1, 2011, trip to a Mission Viejo Best Buy would jeopardize his freedom and eventually raise concerns about, at a minimum, FBI competency or, at worst, corruption. Unable to boot his HP Pavilion desktop computer, he sought the assistance of the store’s Geek Squad. At the time, nobody knew the company’s repair technicians routinely searched customers’ devices for files that could earn them $500 windfalls as FBI informants. This case produced that national revelation.
I have no issues with authorities being alerted by Geek Squad if they find something suspicious in the course of their daily work. After all it is similar to me calling the police if I see someone trying to break into my neighbors house (I would just not expect to get a bonus payment for doing so).
The concern I have here is about payments and possibly other motivating factors (such as political) that may cause a technician to plant evidence on a user machine.
Are there any strategies (from technical perspective) that could be used to prove ones innocence against evidence planted on a customer's computer, or is that an unnecessary concern?