It is worth noting that we can also see this at the police level, where a cop plants evidence, turns on the bodycam recorder, then "discovers" the evidence, getting caught at it only because he didn't fully understand what the record button did.

With retail store employees, there isn't even an expectation of trustworthiness when it comes to handling evidence. They are not officers of the court, and have no duty to act in the interests of justice. They likely do have a duty to the customer to keep their private affairs private.

There are few reasons for any repair technician to ever open files containing customer data, and zero reasons for them to do it outside of a well-sandboxed, auditable environment, with specialized applications rather than the default OS tools. Rather than opening as a rectangular array of pixels, an image would display its bytes in a hex editor, with decoder plug-ins displaying the header information and metadata for that file type, and the results of a file integrity and malware scan.

It seems likely that Geek Squad is not set up for that kind of work. It's just too expensive to sell to consumers in a retail store. So if Joe Employee loads a suspicious image onto the customer's machine, then "discovers" it by opening it, and reports the discovery to the cops for monetary reward, that is indistinguishable from Joe Employee accidentally discovering genuine criminality, because it was not done in an auditable environment.