In grad school, while working with a large scientific collaboration, I would regularly get emails from our software liaison regarding security. One night while working a shift at the experiment our shift leader asks who has screen sharing, file sharing, and printer sharing enabled, all things I turned on to work with the machines on my home network? It was then I realized how easy it was for people to see what ports a machine has open to begin looking for ways to access the machine, and that I should probably start taking security a little more seriously. When it comes to data, it's generally best to prepare for the worst and hope for the best. Once you give someone physical access to your machine, assume it is game over, and that your deepest darkest secrets or most sensitive data have been compromised. If they want it, they can most likely get it at that point. If you have sensitive data stored in cloud storage, be it Drop Box, iCloud, Google Drive, etc., assume it is accessible by someone, whether it's the data on the physical drive or data en route from one place to another.
Practice safe computing practices, use password protected screen savers and sleep modes and encrypt your drive. This will go a long way to keeping the honest people honest. If you need to grant physical access to the machine, remove any sensitive data, especially if they need admin access to the machine. Depending on the sensitivity of the data, it may be worth swapping the drive entirely.
As mentioned, if you are interesting enough for someone to dig through your sensitive data, then you probably know it, and should do your best to lock things down. If you are not, assume that the tech may snoop around anyway just for kicks and giggles.