Here’s my Marble Hornets tag if you want look through it.
The one that I definitely know he’s in, just off of the top of my head, is where Tim (I believe) is running lines in a corner of a room, and there is a window behind him. It’s early on in the “series.”
But if you want to be a trooper and find Slendy yourself, rewatch MH a bunch of times. I’ve seen it three our four times each time I wanted to catch up (because seriously, I forget what goes on and rewatching it all in order helps).
I don’t post MH anymore, because no one really seemed to care, but there are links to it in my creepy links page.