Has it been ever possible to implant a monitoring device or software in a Macbook that cannot be detected by antivirus? I have a several-year old pre-unibody Macbook and am wondering if my family member could have ever put some kinda tracking or monitoring device or software in it either directly or by remote access? I don't exactly know how they would do such a thing. I recently ran Norton antivirus with nothing found, but can it miss some things sometimes? For example i am concerned about it being GPS-tracked or my computer activity being monitored by unknown screen sharing or keyboard tracking ... etc etc
Yes. It is possible, including through Mobile Device Management (MDM) applications which are generally legitimate, and through more nefarious means as well. Since it's such a broad topic, addressing the many ways that someone could do this would be too open-ended. Think of how a school is able to track a computer it has issued to a student, in terms of location, screen-watching, and the sort.

To guarantee, at least from the software side, that this is not happening, you'd want to perform an Erase and Install. In terms of any sort of hardware implementation, that is such an unlikely scenario that I'd count it out.

If you want to check for MDM, most often they will install a Profile onto your computer. This can be seen in System Preferences -> Profiles (fourth row down). It will only appear if a profile is installed.