There soooo many ways!!!
 If anyone needs personal help on this- [email protected]

To investigate this you need to figure out wether  this person is aware of ur Fb activity done on computer or mobile device. If it is done on mobile device then are you using Facebook app or a browser? Always provide more technical details in order to be helped.
  A. If u think your mobile device activity is only known to this person:

1. 
My number # 1 guess is that you have spy app installed. It takes 5-15 min to install if this person has access to your device.  This works only if you have either iPhone or rooted android (if that's not you case then go to #2).
Mobilespy for example. Impossible to detect. In this case you need to restore factory setting on your phone. 
   A little about mobile spy:
    (In this case this person not only can know what and when you viewed on Fb, but also know all your texts, calls, browsing history.. This app is so powerful that gives  another person more controls over your device then you- recording surrounding sounds, viewing surrounding through ur camera (I have used this, this is all possible, and believe it or not but this is even legal).
  2. 
Check wether in any of your browsers or google apps you have someone else's account logged in. Nowadays Google provide a great tool that tracks your web and app activity.
  3.  
Check wether you have any pairing apps installed (pairs mobile for example with iPad to store data and whatever you see on the screen of your mobile can be seen on iPad).

4.
Your Fb app logs is stored on your device and once connected to pc it can be retrieved with special apps.

      B. If your PC Fb activity is known to someone else.
  1. Does this person has access to your computer? 
Browser history provides detailed info on which photo and what time you looked at... And even has your passwords stored..
   2. Sometimes even clearing browsing history will not help (if you are dealing with someone like me), as logs can be retrieved, and not only your Facebook activity, but simply all type of activity done on your computer can be easily retrieved. 
     There are soooo many ways to get as we believe "protected info". If you are dealing with a jealous gf, then you better prepare for the worst:) 
  I can help to analyze the security leaks for free, just out of my personal interest : [email protected]