Here is my 100% Achievement guide and full walkthrough!
This is story related so cannot be missed.
After you talk with Sam in the garage, you then go upstairs to his living quarters/office.
(Note, there is a Collectible on his desk which is on your left as you enter).
Once done here, talk to Sam, and after the ensuing fun and cutscene.. achievement unlocks
58:55 in the vjdeo to see where it does unlock
Hope this helps!