To me, secretively installing any malicious software in any system is essentially “hacking”. You don’t necessarily have to modify the operating system on any platform to install, and bury or hide, spyware, bots, root kits, keyboard loggers, etc, but I still think of that as “hacking“. If you have or gain root or admin access, you can cleverly install all manner of things, and hide your tracks very well, invisible to all but the most technically knowledgeable.
And that of course is at the crux of the OP’s quandary. If someone knows what they are doing (and the owner/user does not) you can never be sure whether something was there, or not. Files can be hidden, processes disguised, logs altered, etc. Unless you are very knowledgeable of the operating system itself, you could not know what had been done, or not done.
If you really suspect something, the only sure thing to do is wipe everything completely, and set up anew from trusted sources only.
And then take basic security steps (like not sharing passwords!) to keep others out! 😉