There is a big difference between seeing drugs on the back seat, or a dead body inside the car, and reporting that, and reporting on drugs found under the carpet in the trunk or in the glovebox if the car was brought in for an oil change...
The mechanic would have had no reasonable need to have searched those two areas to perform the job he was hired to do. Same with a PC tech, if someone brings in a PC to have a CD-ROM drive replaced, there is absolutely NO REASON for the tech to need to search the browser cache or the images directory...
The problem is, because there are different standards of service, what you you've purposed a construction that's beyond what the law and judges can apply equally. Each machanic does different things to the vehicals they're working on and because of that there would be different expectations as to what is private and what is not. A forgotten bag of weed under the seat? Oh, as part of your oil change service, we vacuum the inside carpet. Found a key of coke under the spare? They may have been inspecting it to see if it was still ok; they wouldn't want you to be surprised by a rotten spare on the side of the highway.
Shift this idea to computers. The cache directories are off limits, how about folders on the desktop named DONT_LOOK_HERE? The content of the system desktop backgrounds directory? Which parts of the system are private and which aren't, and how to you apply this equally? This is why you either abandon your expectation of privacy or you don't. If you turn your property over to a third party, you have abandoned any expectation you have in relation to that property.
As for not doing a filesystem search during a cdrom install, if I'm a pc tech, I'm going to run the standard diagnostics on each and every machine that enters my shop for two reasons. First, 90% of the machines I'm going to see are infected with something and I can't ethically allow that machine to leave the store in that state. Second, of that box has a ram problem, I want to know about it before I put a screwdriver to the case. It's not unreasonable to assume that a diagnostic scan is going to alert to a pile of suspiciously named image files in an obscure directory.
