It can be found inside the Kanawha Nuka-Cola plant, in the first room entered from a roof hatch, behind the large console.
My advice to you is hide the thing. Marketing sees EVERYTHING, especially where their money has gone, and there is going to be a budget related firestorm... which lands squarely on you! And me for that matter since I'm about to help you.
I know a place you can stash it, out of sight out of mind. I'll adjust the lines on the expenditures sheets and no one will be the wiser. Don't take any chances, for both our sakes.
I'll take one key with me to Tanagra when I visit the kids. I've left the other one for you. It's in that place where I put that thing that time.
Behind the scenes
The last sentence of the note, "It's in that place where I put that thing that time," is a reference to the 1995 film Hackers. The character Ramόn Sánchez says the same thing while making a phone call from prison.