Not to mention the fact that
a) Slendy doesn't show up until right after he calls Alex to say "Hey dude, I'm alone in the woods...
b) He stated that Alex is acting funny...
c) Masky goes right past Jay, who is surprised and unarmed and knifes Alex leading me to believe that they are on two opposite sides
d) Alex's car is in the parking lot when Sinclair is looking at the cars.
Furthermore, how does Sinclair recognize not only Jay's car but also Jay's camera. Unless, perhaps he's seen the car before, and the camera used to be his?
Lastly I contest that staying on film is a bad idea; draws slendy closer. So, giving someone who is being Slendy stalked, a camera (which both Alex and now Sinclair have done) is not the nicest thing you could do for a guy.
P.S. I really hate dolls. This whole video was me yelling "Awwww....fuck you...aww God Damn..." every time that stupid doll came crawling at me.