Hypothetical Remote Desktop Access

This whole scenario is hypothetical.

Suppose someone has a MacBook Pro manufactured in September of 2013. It runs OS 10.8.5. Would it be possible for someone (using Windows or Linux) who has only the administrator password to remotely stream the desktop of this machine and also the webcam? If this is possible, how would it be done, and what evidence would it leave?

In this hypothetical scenario, the computer has been taken to a Mac subsidiary store and examined by the tech support people there, and nothing of note happening with the computer. It was also seen by a third party who also found nothing of note.

Hypothetically, a program called Little Snitch has been installed on the machine and does not show anything unusual happening in terms of access to the internet when examined by these Mac-savvy people.

Another thing of note: the computer was completely wiped, the OS reinstalled, between the tech support people examining it and the other guy taking a look at it.

Did the suspected person also have access to your home router? You'd have to know in advance which "port number" to use and set that to be "forwarded" in the router.
Your router provides a large part of your security/protection.

On the other hand, if this is within a company's local network, it would be fairly simple. Look in System Prefs > Sharing, and turn off Remote Login and Remote Management.

On the web, read about Remote Desktop and VNC, common methods of controlling another computer.